T 1951/22 26-09-2024

I. Both the opponent and the patent proprietor filed an appeal against the interlocutory decision of the opposition division finding that European patent No. 2 839 169 as amended according to auxiliary request 1.9 met the requirements of the European Patent Convention.

II. The patent results from European patent application No. 13777637.3, filed as International patent application No. PCT/US2013/037393 which was published as WO 2013/159008 A1.

III. The opposition had been filed against the patent as a whole on the basis of the grounds for opposition under Article 100(a) together with Article 54(1) EPC (lack of novelty) and Article 56 EPC (lack of inventive step), under Article 100(b) EPC and under Article 100(c) EPC.

IV. In the decision under appeal, the opposition division came to the following conclusions:

- the subject-matter of claim 11 of the patent as granted extended beyond the content of the application as originally filed,

- the subject-matter of claim 11 of auxiliary request 1 lacked clarity and extended beyond the content of the application as originally filed,

- auxiliary requests 1.1, 1.2, 1.4 to 1.7 and 1.9 were admitted into the proceedings,

- the subject-matter of claim 11 of auxiliary request 1.1 was clear but extended beyond the content of the application as originally filed,

- the subject-matter of claim 11 of auxiliary request 1.2 extended beyond the content of the application as originally filed,

- the subject-matter of claim 1 of auxiliary request 1.4 was not new with respect to document D26,

- the subject-matter of claim 1 of each of auxiliary requests 1.5, 1.6 and 1.7 lacked clarity and extended beyond the content of the application as originally filed,

- the subject-matter of claim 1 of auxiliary request 1.9 fulfilled the requirements of Article 123(2) and (3) EPC and Rule 80 EPC, it was novel and involved an inventive step with respect to the available prior art, in particular document D26 in combination with the common general knowledge or with one of documents D24 and D8.

V. The documents considered in the decision under appeal are the following.

D8: |DE 103 55 250 A1 |

D24:|JP 11-270513 A and its translation (D24a)|

D26:|WO 03/016851 A1 |

VI. Together with its statement of grounds of appeal, the opponent (appellant I) filed the following document.

D30:|Goharrizi and Sepehri, "A Wavelet-Based Approach to Internal Seal Damage Diagnosis in Hydraulic Actuators", IEEE Transactions on Industrial Electronics, Vol. 57, No. 5, May 2010|

VII. With its statement of grounds of appeal, the patent proprietor (appellant II) filed six auxiliary requests. With its reply to the opponent's (appellant I's) statement of grounds of appeal, it filed four further auxiliary requests.

VIII. In a communication pursuant to Article 15(1) of the Rules of Procedure of the Boards of Appeal (RPBA) issued on 26 July 2024, the parties were informed of the board's provisional opinion on the issues of the appeal.

IX. By letter dated 12 August 2024, appellant II filed an eleventh auxiliary request.

X. Oral proceedings before the board were held on 26 September 2024. During the oral proceedings, appellant II reordered its auxiliary requests.

XI. Appellant I (opponent) requested that the decision under appeal be set aside and that the patent be revoked. It was also requested not to admit auxiliary request 1.9, admitted by the opposition division during oral proceedings, and not to admit the second, third, fourth and sixth auxiliary requests into the appeal proceedings.

Appellant II (patent proprietor) requested that the decision under appeal be set aside and the patent be maintained as granted (main request) or, alternatively, the patent be maintained in amended form on the basis of the claims of one of the first to sixth auxiliary requests filed with the statement of grounds of appeal or, alternatively, on the basis of the claims of the eleventh auxiliary request filed with letter dated 12 August 2024 or, further alternatively, on the basis of the claims of one of the seventh to tenth auxiliary requests filed in reply to the opponent's statement of grounds of appeal. It was also requested not to admit document D30 into the appeal proceedings.

In the following, appellants I and II will be referred to as the opponent and the patent proprietor, respectively.

XII. Independent claims 1 and 11 of the patent as granted (corresponding to the patent proprietor's main request) have the following wording (the feature numbering used by the board appears in square brackets):

"[1.a] An actuator system comprising: [1.b] a piston-cylinder arrangement including a piston (40) that is movable with respect to a cylinder (17); [1.c] a first flow path in fluid communication with the piston-cylinder arrangement; [1.d] a second flow path in fluid communication with the piston-cylinder arrangement; [1.e] a control system (30) operable to fluidly connect the first flow path to a source of high-pressure fluid and to connect the second flow path to a drain to move the piston (40) in a first direction; [1.f] a pressure sensor (20) fluidly connected to the first flow path and operable to measure sufficient pressure data during the movement of the piston (40) to generate a pressure versus time curve, [1.g] the control system (30) operable to compare the generated pressure versus time curve to a known standard pressure versus time curve stored in the control system (30) to determine the condition of the piston-cylinder arrangement, [1.h] characterised by the actuator system further comprising a second pressure sensor (25) in fluid communication with the second flow path and operable to measure a second set of pressure data during movement of the piston (40), [1.i] and the control system (30) being operable to compare the second set of measured pressure data to a second known standard to determine the condition of the system."

"[11.j] A method of predicting a failure in an actuator system, the method comprising: [11.k] porting a high-pressure fluid to a first side (50) of a piston-cylinder arrangement; [11.l] draining a low-pressure fluid from a second side (55) of the piston-cylinder arrangement to allow the piston (40) to move with respect to the cylinder toward the second side (55); [11.m] taking a plurality of pressure measurements of the fluid adjacent the first side during the movement of the piston (40); [11.n] taking a plurality of measurements of the fluid adjacent the second side during movement of the piston (40); [11.o] comparing the plurality of pressure measurements to a known set of pressure values; and [11.p] determining if a failure is likely based on the comparison of the plurality of pressure measurements to the known set of pressure values."

XIII. The wording of claim 1 of each of the first, third, fourth and fifth auxiliary requests is identical to that of claim 1 of the patent as granted.

XIV. Independent claim 11 of the second auxiliary request has identical wording compared to claim 11 of the patent as granted.

XV. The sixth auxiliary request has, in addition to independent claim 1, a second independent claim relating to an actuator system.

XVI. The eleventh auxiliary request has a single independent claim with, compared to claim 1 of the patent as granted, the following additional feature.

"[1.j] comprising a piston seal (60) coupled to the piston (40) to inhibit fluid flow between the piston (40) and the cylinder (17), the control system (30) operable to predict a failure of the piston seal (60) based on the comparison of the generated pressure versus time curve to the known standard pressure versus time curve stored in the control system (30)."

XVII. The opponent essentially argued as follows.

Main request - Ground for opposition under Article 100(c) EPC

The subject-matter of independent claim 11 of the patent as granted extended beyond the content of the application as filed. The term "pressure" did not appear in feature 11.n. At the same time, the wording of feature 11.n was not ambiguous and did not require any interpretation. It did not involve an obvious error either. Given that paragraph [0045] of the published application as filed also referred to additional sensors, it was incorrect to read the term "pressure" into the term "measurements" of feature 11.n. Claim 17 of the application as filed only provided basis for taking pressure measurements. When assessing the disclosure of the application as filed as a whole, the skilled person would only ever find a reference to pressure measurements and not to the more general "measurements". Therefore, feature 11.n was not originally disclosed. The patent proprietor's argument that the skilled person would exclude technically illogical interpretations was not objected to; it actually supported the decision of the opposition division in respect of an inadmissible broadening of the technical expression "pressure measurement". That viscosity measurements were technically illogical was an unsubstantiated assertion by the patent proprietor.

First auxiliary request

- Novelty

The subject-matter of claim 1 of the first auxiliary request was not novel over the disclosure of document D26. As regards features 1.e and 1.g, the control system of document D26 was formed by the valves 13 controlling the actuator 3 and by the evaluation unit 6 designed as a PLC controller. The patent proprietor was wrong in asserting that, in the absence of a functional or structural connection, these had to be separate control systems. The broad wording of claim 1 of the first auxiliary request also included a control system with two subsystems. Figure 1 of the patent, for example, illustrated a control system 30 without any connection to the valve 70. Furthermore, the patent proprietor's argument that features 1.g and 1.i were not disclosed by document D26 could not be understood. It followed from page 10, lines 30 to 31 of document D26 that the prior art actuator was a double-acting cylinder. Therefore, the condition of the pressure lines 1, 2 shown in Figure 4 of document D26 also allowed conclusions to be drawn on the pressure in the two piston chambers of the actuator with which they were connected. Given that claim 1 of the first auxiliary request was a device-type claim and that the prior art system of document D26 was operable to determine the condition in every possible mode of operation (see page 6, last paragraph of document D26), it did not matter whether the pressure lines were connected to the high-pressure or low-pressure side of the actuator. Moreover, it had to be taken into account that feature 1.i did not refer to the actuator per se but to the actuator system.

- Inventive step

It would have been obvious to unite the control systems of document D26 into one control system. Therefore, the subject-matter of claim 1 of the first auxiliary request did not involve an inventive step.

Second to sixth auxiliary requests

The second, third, fourth and sixth auxiliary requests were not subject of the oral proceedings held before the opposition division; they were filed for the first time in the appeal proceedings. However, they could have easily been filed in the oral proceedings held before the opposition division. In addition, the problems of added subject-matter concerning independent claim 11 as granted were not resolved in the second auxiliary request. The second, third, fourth and sixth auxiliary request should therefore not be admitted into the appeal proceedings.

The fifth auxiliary request corresponded to auxiliary request 1.4 submitted before the opposition division. Claim 1 of the fifth auxiliary request was not new compared to document D26 for the same reasons as presented in the context of the first auxiliary request.

Eleventh auxiliary request

- Admittance into the appeal proceedings

The eleventh auxiliary request, which was identical to auxiliary request 1.9 underlying the decision under appeal, must be rejected as an inadmissible late submission. The opponent disagreed with the opposition division's conclusion that claim 1 of auxiliary request 1.9 contained a foreseeable amendment. In all auxiliary requests filed throughout the opposition proceedings, the patent proprietor had pursued protection for a system that aimed to determine the condition of the piston-cylinder arrangement without, however, explicitly naming the component involved. When the patent proprietor filed auxiliary request 1.9 containing the limitation to piston seal at 17:35 during the oral proceedings before the opposition division, it had come as a surprise. Also, the filing of auxiliary request 1.9 could not be regarded as a reaction to the opposition division's reconsideration of novelty over document D26, since this had been discussed in the context of auxiliary request 1.4. The patent proprietor had been allowed three attempts to respond to this change of view. In none of these attempts it had included the aspect of the piston seal in independent claim 1. Therefore, the patent proprietor had been in a position to file auxiliary request 1.9 at a much earlier stage. The patent proprietor should not have been allowed to first file a number of auxiliary requests with amendments based on features taken from the description, which therefore had a lower chance of success, and, only after all of these attempts had failed, to combine original claims 1 and 2 of the patent as granted in auxiliary request 1.9.

- Admittance of document D30 into the appeal proceedings

The restriction to the aspect of the piston seal in claim 1 of auxiliary request 1.9 had been surprising to the opponent, at the time it was filed at the oral proceedings before the opposition division. Since it was no longer possible for the opponent, in view of the late hour, to react appropriately during these oral proceedings, document D30 should be admitted in the appeal proceedings. This prior-art publication filed at the outset of the appeal proceedings was not purely theoretical but dealt with the function monitoring of concrete actuator systems. In line with the background described in paragraph [0002] of the patent, page 1755, first column, first paragraph of document D30 referred to condition monitoring of hydraulic systems and to early detection of faults. This implied a failure prediction. As was clear from Figures 1 and 2 of document D30, an experimental test rig was disclosed, which comprised an actuator in the form of a double-acting working cylinder, two fluid paths and a control system in the form of a high-speed PC. Further reference was made to the abstract on page 1755, to page 1756, second column, third paragraph, to page 1757, second column, second paragraph mentioning the piston seal, to Figure 5 according to which so-called 'baseline' pressure-versus-time curves were recorded for both piston chambers, to Figure 7 which showed an internal leakage signal, to Figure 8 which showed pressure-versus-time curves recorded during the internal leakage originating from a leaking piston seal, and to page 1760, left column, second paragraph regarding the comparison of the curves. In this way, document D30 explicitly disclosed a function monitoring system for a piston-cylinder arrangement in which it was possible to predict the failure of a piston seal and, thus, prima facie opposed the maintenance of claim 1 according to the eleventh auxiliary request.

- Inventive step

The subject-matter of claim 1 of the eleventh auxiliary request did not involve an inventive step. Starting from document D26, feature 1.j was a further distinguishing feature resulting in the objective technical problem of providing an actuator prediction system with which a fault prediction on a specific component of a piston-cylinder arrangement was possible. The skilled person would have arrived at the claimed actuator system either in view of common general knowledge alone or in combination with document D24 or document D8.

As shown by Figure 4 of document D26, the piston-cylinder arrangement had a cylinder housing in which a piston connected to a piston rod was movably guided, hereby separating two pressure chambers A and B from each other. Not only the pressure in the supply lines but also the pressure inside the piston chambers was monitored. It would have been obvious to the skilled person that, if no or only little pressure had built up on the inlet side and the actuating piston did not move, this was not necessarily due to a leak in the pressure line, but possibly also to a faulty piston seal. On page 7, lines 19 to 25 of document D26 it was explicitly stated that further parameters could be included in the monitoring profile. Thus, the skilled person would have taken the suggestion from document D26 to extend the function monitoring to the piston-cylinder arrangement and, in particular, to include the piston seal.

Alternatively, the skilled person would have taken document D24 into consideration as it also disclosed a double-acting actuator with a function monitoring system. In lines 25 to 27 of document D24a, the prediction of the deterioration of a fluid actuator was mentioned. In Figures 1 and 3 of document D24, a piston seal 26a was shown, see also line 112 of document D24a. The control system 10 of document D24 was operably connected to both the pressure sensors 14A, 14B and the control valve 50. Figures 5 to 9 of document D24 showed pressure-versus-time curves. In line 198 of document D24a, reference was made to the worn state of the piston seal. The expression "wind force" corresponded to the pressure while also taking into consideration the fluid flow, similarly as in document D26. As a consequence, the skilled person would have found suggestions in document D24 to use a single control system and to predict also the failure of the piston seal. Applying this to the piston-cylinder arrangement of document D26, which implicitly comprised a piston seal in order to prevent fluid from leaking in between the chambers, would have resulted in the claimed subject-matter.

Furthermore, document D8 was concerned with a method for determining leakage in an actuator system with a piston cylinder. As Figure 2 of document D8 in particular disclosed, the piston cylinder had a piston 3 which separated two pressure chambers 10, 11 from each other. It would have been obvious to the skilled person in view of the teaching of document D8 to apply the function monitoring described in document D26 also to the piston cylinder.

XVIII. The patent proprietor essentially argued as follows.

Main request - ground for opposition under Article 100(c) EPC

The subject-matter of independent claim 11 of the patent as granted did not extend beyond the content of the application as originally filed. It was established jurisprudence that the skilled person interpreted a claim with a mind willing to understand and that technically illogical interpretations should be excluded. A reading of feature 11.n in the sense that it encompassed measurements of a general kind, and not specifically pressure measurements, was technically illogical. The subsequent mentions of measurements in features 11.o and 11.p specifically referred to pressure measurements and relied on pressure measurements being taken on both sides of the piston. The skilled person would realise that it did not make technical sense to take measurements which were not used subsequently in the method and would, as a result, never interpret the claim in this way. Otherwise the taking of measurements adjacent the second side of the piston would be for apparently no reason at all. The technical teaching of the disclosure as a whole was specific in that a comparison of pressure curves enabled the determination of the condition of the actuator. There was, however, no teaching in the patent as to how a comparison of viscosity measurements could be used to determine the condition of the actuator, nor for that matter any other kind of measurements. It was obvious that feature 11.n contained a typographical error and should read "pressure measurements". That this had always been intended followed from points 5 and 6 of the applicant's letter dated 11 July 2016. Source of the error was probably the mention of "second sensor" in paragraph [0030] of the published application as filed. The wording used in paragraph [0044] of the application as filed clearly demonstrated that other measurements were only considered in addition to the pressure measurements. Feature 11.n could thus not be understood in the sense that the temperature or viscosity were measured adjacent the second side of the piston. As such, basis for claim 11 as granted was provided by original claim 18 in combination with paragraph [0036] of the application as filed, which described taking pressure measurements adjacent first and second sides of the piston, as well as paragraphs [0050] to [0055] of the application as filed, which provided a detailed explanation of a comparison of pressure measurements taken adjacent both sides of the piston being compared and used to determine a condition of the system.

First auxiliary request

- Novelty

The subject-matter of independent claim 1 of the first auxiliary request was novel over document D26, which failed to disclose features 1.g, 1.h and 1.i. Firstly, claim 1 of the first auxiliary request required a single system to be responsible for both controlling and evaluating the operation of the actuator. It was clear from the claim wording that any subsystem which performed the comparison and determination steps must have some kind of functional or structural link with another subsystem that had a role in controlling the actuator. Otherwise, they could not logically be described as forming part of the same control system. Document D26, however, provided a separate purpose-built evaluation unit 6 which had no connection whatsoever to the mechanisms which controlled the piston's movement. The two control systems of document D26 did not share any data or have any common processing or logic. It therefore did not make technical sense to describe the evaluation unit 6 as forming part of the control system of feature 1.e. Secondly, the skilled person would understand from the wording of features 1.g and 1.i that the control system itself had to be operable to determine the condition of the actuator system. The functionality of the control system was described throughout the published application as filed (see paragraphs [0030] to [0033], [0048] and [0062]). The idea that an individual be charged with watching an output monitor displaying graphs of output data and using that information to determine the condition of the actuator system was non-sensical. In reality, the control system of the present invention would be employed to monitor many tens if not hundreds of actuators in a factory setting, which were each operated many times a minute, sometimes over 24 hours. Document D26 did not disclose a system which determined a condition of the actuator system itself. Instead it disclosed a system which determined the presence of leaks in supply lines connected to an actuator system (see, for example, page 1, lines 4 to 9 and page 6, lines 24 to 29 of document D26). Thirdly, document D26 did not explain how the actuator was controlled to operate in the reverse direction. Sensors 4 and 5 were used to detect preload and operating pressures, respectively. This would be on the high-pressure side of the piston. It was unclear whether the sensors were operable to measure pressure on the low-pressure side of the piston, as required by feature 1.h.

- Inventive step

The subject-matter of claim 1 of the first auxiliary request involved an inventive step. The pressures measured by the evaluation unit 6 of document D26 were constantly compared to threshold values. In the last two paragraphs on page 13 of document D26, it was explained that the purpose of the unit 6 merely was to look at a screen. There would be no efficiency gain in combining the valve control and the monitoring action of document D26. Furthermore, it was not clear which data or logic would need to be shared between the operation of the valves and the pressure measurements. It followed from paragraph [0031] of the patent that the control system of the invention also served to more directly control the operation of the actuator. The skilled person would not have been motivated to carry out such a change in the actuator system of document D26.

Second to sixth auxiliary requests

Independent claim 1 of the second auxiliary request was amended to reflect that the control system was operable to determine the condition of the actuator system. Independent method claims 11 of the third and fourth auxiliary requests were further amended in order to comply with Article 123(2) and 84 EPC.

The fifth auxiliary request comprised the apparatus claims of the main request. Method claims 11 to 13 were deleted. The arguments presented for claim 1 of the main request therefore also applied to the fifth auxiliary request.

The sixth auxiliary request comprised claim 1 as maintained in opposition proceedings, corresponding to claim 1 in combination with dependent claim 2 as granted. A new independent apparatus claim 2 was added consisting of independent claim 1 in combination with dependent claim 3 as granted. Method claims 11 to 13 were deleted.

Eleventh auxiliary request

- Admittance into the appeal proceedings

The eleventh auxiliary request should be admitted into the appeal proceedings. It was identical to auxiliary request 1.9 on the basis of which the patent was maintained by the opposition division. This request, therefore, was not a novel or unforeseen development in the appeal process. Auxiliary request 1.9 had resulted from a combination of claim 1 with dependent claim 2 as granted, and was therefore a request which could have easily been anticipated by the opponent. Furthermore, it had been filed in reaction to the reconsideration of the opposition division's preliminary opinion. It was established jurisprudence of the Boards of Appeal of the EPO that opponents should be prepared for amendments in which an independent claim as granted was limited by the features of a dependent claim as granted. Actually, the opponent had already taken the opportunity to challenge claim 2 as granted in its notice of opposition. The opponent thus came into oral proceedings before the opposition division pre-armed with attacks to auxiliary request 1.9. It was therefore clearly absurd to suggest that the opponent could not have anticipated this request. In addition, auxiliary request 1.5, which had been submitted prior to the Rule 116(1) EPC deadline, related to the same subject-matter as auxiliary request 1.9, namely to predicting/identifying a failure of the piston seal. Auxiliary request 1.9 had only been the patent proprietor's second opportunity to address a surprise finding of lack of novelty of claim 1 in the context of auxiliary request 1.4. Auxiliary request 1.5 was rejected based on the formal issues of added subject-matter and lack of clarity that were first presented to the patent proprietor very late in the day during the oral proceedings. Accordingly, it was only fair that the patent proprietor was given the opportunity to react to a surprise change of mind by the opposition division.

- Admittance of document D30 into the appeal proceedings

Document D30 should not be admitted into the appeal proceedings. Auxiliary request 1.9 was based on a combination of claims 1 and 2 as granted. Thus, the opponent would readily have anticipated the subject-matter of auxiliary request 1.9 and ought to have conducted all searching for prior art to attack this claim prior to filing the notice of opposition. The opponent therefore had had multiple opportunities to submit document D30 during first-instance proceedings and had not fulfilled its obligation to submit all facts and arguments as early and completely as possible. Furthermore, as several features of claim 1 of the eleventh auxiliary request were missing from document D30, it failed in meeting the threshold of prima facie relevance. The paper merely verified whether, based on wave decomposition in response to a pressure pulse, a value of the pressure signal fell below a threshold. The pressure-versus-time curve was thus not related to the movement in a first direction as required by feature 1.g. According to the abstract of document D30, the pressure was only measured at one side of the actuator, contrary to the requirement of feature 1.i. And there was no disclosure in document D30 of identifying or predicting an actual failure; merely the response to any kind of generated leakage signal was detected. Also feature 1.j was thus not disclosed. In conclusion, it was evident that document D30 was less relevant than other documents already relied on during the opposition proceedings, such as document D26.

- Inventive step

The subject-matter of claim 1 of the eleventh auxiliary request involved an inventive step.

Feature 1.j required that failure of the piston seal was predicted based on the comparison of the generated pressure-versus-time curve with the known standard pressure-versus-time curve stored in the control system. This would not have been obvious for the skilled person starting from document D26. Even if document D26 described how parameters other than pressure additionally enabled monitoring of the connected unit, this was not connected to a comparison of pressure-versus-time curves. Regardless of whether the skilled person would have monitored the piston-cylinder arrangement of document D26, there was no teaching in document D26 which would have motivated the skilled person to diagnose a particular fault as occurring at a specific component, specifically, the piston seal. The opponent did not provide any reasoning as to how the skilled person minded to use the method to assess the condition of the cylinder, might have been able to actually identify specific problems with the piston seal.

Document D24 disclosed a method for detecting abnormalities of a fluid actuator in which pressure was measured in first and second ports. Volume flow rate of fluid flowing into the first and second ports was also measured using a flow meter. The measured pressures and the flow velocity were then integrated to obtain a value proportional to the force applied to the fluid actuator, see lines 67 to 74 of document D24a. This calculated value, referred to at various points as the "wind force", was then used to indicate defects in the system. Thus, in document D24 there was no comparison of pressure values as required by claim 1. The skilled person attempting to identify defects in the actuator would therefore not have had any reason to combine documents D26 and D24. If at all, the skilled person, after having consulted document D24, would have attempted to identify defects occurring specifically in the actuator by comparing a calculated wind force.

Document D8 disclosed a method for leak detection in an actuator which involved measuring pressure and volume flow in one of two supply lines during movement of a moving element of the actuator. The measured pressure and volume flow were used to calculate the position and speed of the moving element for the leak-free condition using a specific equation in which the coefficients were calculated by modelling the system in the leak-free initial condition. Deviations between the calculated position and speed of the moving element were used to determine leakage. It was, however, not clear how the skilled person would have combined the disclosures of documents D26 and D8. Any combination would have altered the principle of operation of document D26. For instance, document D8 taught to measure different parameters to those in document D26. Document D8 further relied on the use of physical relationships to model different variables and involved the calculations of several parameters and coefficients. There was no suggestion in document D8 that leakage could be identified within the cylinder by comparing a measured pressure curve with a standard pressure curve. The skilled person trying to adapt the method of document D26 to enable detection of leakage in the cylinder, as in document D8, would thus have been forced to implement the methodology of document D8, contrary to what was specified in claim 1. Furthermore, it was noted that the opponent did not even attempt to assert that the skilled person could have specifically identified a failure of the piston seal.