A wet clutch (as opposed to a dry clutch) is bathed in a cooling, lubricating fluid to protect both steel and friction plates from wear. Wet clutches are ideal for high torque applications, which include motorcycles and several types of transmissions in passenger cars and heavy-duty vehicles.

Four-stroke motorcycles are one of the most common vehicles that use a wet clutch system. Most motorcycle transmissions employ a multiple-plate wet clutch, which stacks clutch discs to compensate for the lower coefficient of friction in a wet system and help eliminate slippage under full power.

Multi-plate wet clutch transmission designs have remained fairly constant over the years, according to Mark Wilkes, small engines global business manager at Lubrizol. But wet clutches in motorcycles are bathed in the same oil as the engine, adding another layer of difficulty for motorcycle oil formulators.

The motorcycle oils are multifunctional and have to work in the crankcase, clutch and the gearbox, Wilkes explained. This means that the hardware benefits from having dedicated formulations to enhance performance in all aspects required for the application. This can include the use of specific componentry in the additive systems and a number of specialty viscosity modifier systems to deliver enhanced clutch friction, gear protection and crankcase performance in the relatively higher-severity motorcycle applications.

In passenger cars, wet clutch systems are typically found only in automatic transmissions, including stepped (planetary) automatic transmissions, dual-clutch transmissions and continuously variable transmissions.

Stepped transmissions are the most common type of automatic transmission, but Infineum expects the worldwide number of vehicles with CVT and DCT designs to surpass those with traditional planetary gear designs by 2024. Regional markets will be fragmented, though, with DCTs seeing the greatest increase in market share in China and Europe, for example.

In North America, light-duty service fill automatic transmission fluid demand will grow 28 percent for CVTs and 47 percent for DCTs, the additive maker predicted in its 2018 Infineum Trends presentation. However, North America is still a slow-growth market for these alternative transmissions. CVT fluids, although experiencing a large percentage growth, will still account for less than 10 percent of the market, with most still being under warranty. The same goes for DCT fluids, which are unlikely to account for more than 2 percent of the North American market by 2021.

Striking a Balance

A basic automatic transmission fluid formulation is composed of 75 to 90 percent base oil, 2 to 15 percent viscosity index improver and 5 to 10 percent additive package, according to Afton. The additive package consists of friction modifiers, dispersants, detergents, protective additives and seal swell agents.

An ATF has multiple functions, even as a dedicated fluid. It acts as a hydraulic fluid in the valve assembly, where it must control foam and aeration, flow at low temperatures and protect the seal. In the gears, it provides wear and corrosion protection. In clutches, it provides friction control and friction durability and dissipates heat.

In wet clutch systems, the fluid must enable the correct ratio of high- and low-speed friction, said Michael Gahagan, dual-clutch transmission fluid formulator for Lubrizol. Too high a friction at low speed can lead to too rapid a clutch engagement and a poor friction response in the clutch. This can lead to noise, vibration and harshness when the clutch pack struggles to engage in an efficient manner, which can impact the overall durability of the clutch itself, he explained to LubesnGreases.

Friction control in a typical power transmission formulation is really about maintaining a level of friction, providing positive friction versus a steep curve, and is usually achieved not only by friction modifiers but also dispersants, and in some cases even detergents are required to bring friction level up, Alexei Kurchan, technical service manager at Evonik Oil Additives USA, explained at the Society of Tribologists and Lubrication Engineers annual meeting last year.

Using too much friction modifier would cause the friction level to drop too low, he cautioned. Further, a friction modifier based on oleic acid could induce oxidation problems or cause sludge formation. The design of the transmission must also be taken into consideration based on the number of gears that need to be lubricated, and the fluid must withstand high temperatures, maintain high friction and maintain cleanliness.

Addressing these challenges requires lubricant formulators to look at the materials used in wet clutch components. While some clutches have direct steel-on-steel contact, many manufacturers coat wet clutch components with paper friction material-porous composite structures that can consist of polymeric binder, graphite or cellulose, aramid (Kevlar), carbon or metal particle fibers.

Paper friction material is used on steel clutch plates because it doesnt follow the normal friction-to-sliding-speed relationship. In a typical steel-on-steel contact Stribeck curve, we show high friction at slow sliding speeds, and as sliding speed increases and more lubricant gets entrained in the contact, friction gradually draws to hydrodynamic lubrication and becomes lower, he said.

By contrast, with a properly formulated power transmission fluid, paper-on-steel contact shows an inverse relationship with the Stribeck curve, noted Kurchan. (See graph on Page 34.) At slow speed friction is low, and as sliding speed increases, friction rises as well. That [Stribeck] curve is actually crucial for a properly functioning clutch to avoid slipping, to avoid shudder and to avoid vibration on the transmission, he added.

Porosity, type of fiber and morphology of the paper friction material on both macroscopic and microscopic scales seem to be contributing factors to this positive speed-versus-friction curve. Therefore, blockage or polishing of the porous material due to wear or glazing (caused by varnish deposits and sediments) would prevent the lubricant from creating this proper friction curve and cause issues in normal clutch operation, said Kurchan.

Testing Clutch Performance

To investigate whether certain viscosity index improvers might be beneficial in ATFs for passenger car transmissions with wet clutch systems, Evonik conducted friction tests using two candidate automatic transmission fluids blended with VIIs based on polyalkylmethacrylates, a common type of VII with strong high-temperature stability.

One formulation was a commercially available ATF with a PAMA used in highly shear-stable applications. A second formulation was blended with a functionalized PAMA and had similar viscometrics and shear stability to the commercially available ATF. An Evonik representative later explained that the functionalized PAMA was designed to make the polymer polar, which improves dispersancy, cleanliness and friction profiles in the finished fluid.

Both test formulations were screened using a Wazau tribometer, which consists of a chamber filled with oil, a friction ring where friction material is added, and a steel ring. This device allows researchers to control sliding speeds, temperature and pressure and to measure torque under steady state loading conditions, said Kurchan.

The oil samples were measured for friction performance at 50 and 110 degrees Celsius at a pressure of 1.1 megapascals, which reflect a range of power transmission temperatures and pressures while in operation. Both PAMAs showed high friction properties at both temperatures.

After this screening, Kurchan sought to measure how the PAMAs would behave in real-world conditions through clutch engagement-disengagement cycles. For this test, they used an SAE No. 2 Universal Wet Friction Test Machine to examine friction durability, following the JASO M348 standard. The machine makes use of dynamic loading engagement to age the oil, which mimics the actions of a shifting clutch.

The test conditions for dynamic friction testing of a full clutch pack included a rotational speed of 3,600 revolutions per minute, oil temperature of 100 C, friction plate surface pressure of 785 kilopascals and 5,000 test cycles of 30 seconds each. Kurchan said that the PAMAs gave an ideal profile of high friction and a flat curve, indicating that harsh, abrupt shifts or slippage of the clutch would be avoided while in operation.

Stability and Durability

A second test to measure static friction, where pressure was applied to the clutch pack, was conducted after 1, 5, 10, 20, 50, 100, 200, 500, 1,000, 2,000, 3,000, 4,000 and 5,000 cycles. Here, the rotational speed was lowered to 0.70 rpm, and the test cycles were run for three seconds. The oil temperature and friction plate surface pressure parameters remained the same as in the dynamic friction testing.

This measurement helps formulators and engineers to define torque capacity, said Kurchan. He added that the torque trace found in the static friction test was also flat, with minimal vibration in the clutch.

Evonik took the results from both sample fluids and measured the durability and stability of the PAMAs in a wet clutch system by defining an S parameter, which subtracted friction results at 200 cycles from the results at 5,000 cycles. Kurchan explained that the results were measured against 200 cycles because that is the point at which the clutch pack seemed to begin showing wear and settling into the transmission.

Results showed that the formulation with the functionalized PAMA maintained better clutch performance compared to the commercially available formulation with the traditional PAMA, showing more stable friction properties. The functionalized PAMA also exhibited better resistance to wear, which was measured by weight loss of the clutch plates.

Kurchan summarized that the functionalized PAMA increased paper-on-steel friction and improved friction durability. Oxidation tests also showed that the same functionalized PAMA can improve sludge dispersancy and part cleanliness.

Future work will focus on adjusting the treat rate of friction modifiers and dispersants, determining if a different formulation approach maximizes paper-on-steel friction, and establishing a longer durability test to determine how the functionalized PAMA performs over 10,000 to 20,000 cycles, Kurchan concluded.