Bonded piston seals deliver performance economies

Jeff Pyle, product engineering manager of Acadia Polymers describes the use of different elastomers in the design of lip seals

Designers of elastomer sealing systems are continually challenged by manufacturers of servos, reciprocating clutches, power pistons and automatic transmissions to improve sealing functionality. Dynamic lip seal designs should improve performance, exhibit greater durability and longer life, and cost less. Just as important, they should weigh less and fit in the same, or less space, than previously required. The ultimate goal is to produce certifiable products that continue to perform during warranty periods and beyond.

Many sealing systems for reciprocating clutches and servos, used in applications such as automatic transmissions, have evolved over the past 50 years (Figure 1). Displacement type seals; O-rings, D-rings, and lathe cut seals were first onto the market. Next came loose lip seals in both short lip and long lip profiles. Both types of loose lip seals are assembled into a metal carrier. Bonded lip type seals, on the other hand, incorporate the lip and metal carrier into one piece.

While all types of lip seals use similarly shaped sealing lips, the basic principles of design are different. Long lip seals, where the sealing force is proportional to the applied oil pressure, minimise the effects of variability on the function of the shifting components. Short lip profiles improve stability in applications where tipping and rolling may be a problem. They are especially effective where centrifugal forces tend to unseal the lip tip; for example, on inverted seals at speeds above 4000 rpm.

For the past 20 years the application of bonded piston seal technology has been limited to lower pressure servo pistons. However, more recently, many designers have been evaluating this sealing technology, especially for reciprocating clutch applications. Either conventional lip seal designs or newly created lip configurations may be bonded directly to either a metal or composite carrier forming a unitised component that does not require subassembly.

Application of bonded piston seals in automatic transmissions dates back to the 1960’s when Allison used them in off-road vehicles. Chrysler was the first automotive user in the A-604, 4-speed transmission. Today, one-piece bonded piston seals are an economic replacement in transmissions either during original design, or as cost-saving retrofits.

How They Work

Sealing is accomplished by creating an interference fit between the sealing lip and the mating component. The required interface is produced in two ways.

First, dimensional interference designed into the sealing system causes the lip to interface with the mating component surface. For example, an outside diameter lip is moulded larger than a bore diameter, and an inside diameter lip is moulded smaller than a shaft diameter.

The second sealing system function is produced by hydraulic pressure which forces the lip against the mating surface. Total sealing effort is a combination of the designed-in lip interface and the pressure being sealed.

One-piece bonded piston seals reduce the number of parts and the time needed to install conventional lip seals. By combining proprietary lip geometries with custom formulated elastomers, the seals are alternatives for servo piston and reciprocating clutch lip seals in automatic transmissions for automobiles, off-road, heavy-duty, marine, agricultural and industrial vehicles. Bonded piston seals are also finding application in pneumatic and fluid power driven devices.

Bonded piston seals provide several benefits to transmission manufacturers. First, there is a reduction in the number of parts that are needed. The bonded piston is one-piece versus the 3 to 5 parts needed when utilising conventional lip seal, O-ring and D-ring designs.

Also, there is the elimination of a subassembly; the one-piece seal simplifies installation procedures, and there is less chance of error and fewer rejects. Leak paths and noise vibration harshness (NVH) are also reduced; the one-piece seal eliminates problematical tolerance gaps that may exist between parts of conventional seals.

Space requirements are reduced too. Bonded piston seals may reduce the amount of space needed, allowing engineers to either add additional clutch plates, or to design more compact transmissions. Bonded piston seals also allow for automated assembly; the bonded piston seals are compatible with automatic assembly machinery.

The basic design of a bonded piston is not complicated. However, attention to several functional areas is crucial. The rubber-to-insert bond is the most critical area, and must assure a long-life component. The demands on high pressure transmission seals are much greater than the demands of typical low pressure applications. Special material and adhesive formulations may be needed to yield the maximum bond strength as well as optimal functional characteristics.

The most effective method of optimising performance has been finite element analysis (FEA). It is crucial that this analysis be performed early in each design.

FEA is a design tool that helps evaluate structural integrity and predict deflection of inserts. Linear FEA analysis is used to design the metal or composite carrier. Non-linear FEA analysis is used for the design of the elastomeric sealing elements.

FEA determines areas of maximum stresses and significant deflections, and identifies safety margins to protect against seal failure due to a combination of factors such as stress concentration and thickness of the insert. A finite element model is constructed using a `slice’ of the insert. If a return spring is to be included, it is constructed to contact the bonded piston seal model on portions of the flanges.

The model also includes restraints, applied perpendicular to the faces of the model to simulate the presence of the entire insert. Fixed vertical restraints are applied to the two surfaces in contact with the return spring to complete the pressure loading envelope within which simulation will occur.

OFF-ROAD VEHICLEs

An example of an analysis of a bonded piston seal in a transmission for an off-road vehicle can be seen in Figure 2. Results demonstrated that insert deflections at significant locations are small and will neither adversely effect sealing compatibility nor cause interference with the return spring.

Environmental conditions in which the seal will perform dictate specific elastomer formulations for an application. The elastomer materials must be fully evaluated in terms of anticipated operating conditions and performance requirements.

Fluid compatibility and operating temperatures are generally the main governing factor for seal material selection. Because of the importance of seal material, all existing data, and the experience of both the user and supplier, must be fully analysed. It is also important that candidate seal materials be evaluated for significant changes in properties in the actual application fluid at normal operating temperatures.

Figure 3 provides a general description of a few typical sealing materials. It shows some advantages and disadvantages for each material. These range from the inexpensive nitrile compounds, to the more expensive fluoroelastomer and polytetrafluoroethylene compounds. In the middle ground lie the ethylene and polyacrylic compounds. Each formulation has its own specific temperature ranges, technical advantages and disadvantages.

Individual manufacturers may offer compound formulations that may differ from these formulations. In cases where the limits of the material are approached, it is important to contact the seal supplier for more detailed specifications on the material.

By bonding the sealing lip directly to the metal or composite carrier, several operating benefits over traditional, `seal-in groove’, technology are realised. Leak paths associated with conventional lip configurations, for example, are reduced from two to one. Since subassembly is not required, seals cannot be installed backwards, or omitted. Also, lip rollover during assembly is eliminated and, since there is no groove, sealing can usually be produced using less axial space.

Servo and accumulator pistons have additional steps associated with the pins and rod required for these applications. Seal carrier design optimisation is the first step in design of a typical clutch, servo or accumulator bonded piston. Envelope restrictions, clutch plate apply-surface requirements and return spring dimensions all typically dictate the shape of the carrier.

Better with fea

Through FEA, the optimum stock thickness for the associated parameters and envelope constraints are determined. Lip dimensions are typically dictated by envelope constraints. Figure 4 shows a drawing of a generic bonded piston lip seal. Application conditions and constraints probably will alter the final design configuration. Also, pressure and temperature requirements, as well as other application parameters and design configurations, will determine the appropriate elastomer material.

Bench testing of piston seals is typically the first step in the validation process. Acadia has developed a unique seal tester that verifies seal material and configuration by putting the seal through its operational paces while it is installed in a customer supplied housing. These tests effectively duplicate application conditions of the final operating environment.

Testing at maximum pressure and temperature can indicate the long term durability of the seal and establish the design feasibility of the configuration. Typical hot and cold cycle testing is performed at temperature extremes ranging from -40 degreesC to 482 degreesC (-40 degreesF to 300 degreesF). Maximum pressure and travel conditions will add to the severity of the test.

The acceptance criteria will vary from application to application, but generally the minimum performance level is to remain functional throughout the test duration. Other areas which may require investigation include installation force analysis and deflection analysis.

Though bench validation testing can not replace dynamometer and vehicle testing, it can be a useful tool in the evaluating material selection, design feasibility and long term durability. Along with the other information, a thorough design analysis can add to the confidence level of the engineer prior to the installation of the seal into a transmission.