Abrasion-resistant, low-friction DuPont™ Vespel® polyimide parts for step-less tractor transmissions
The German gearbox specialist ZF Passau GmbH uses extremely abrasion-resistant and low-friction DuPont™ Vespel® thrust washers for its step-less, hydrostatic power-split Eccom CVT transmissions. The transmissions are deployed in agricultural machinery from internationally renowned OEMs such as John Deere, Claas Selbstfahrende Erntemaschinen or Same Deutz-Fahr. The planet wheels rotate against the washers separating them from the planet wheel carrier, thereby ensuring the consistently reliable and low maintenance operation of the transmission. In addition, rectangular cross-section rings are used on the drive shafts of the higher-rated transmissions. Overall, Eccom transmissions may contain over 20 Vespel® parts, all of which are produced by DuPont.
Vespel® thrust washers: minimum friction – less abrasion
Vespel® SP parts are suitable for continuous use temperatures of up to 290 °C, as well as short-term peaks of up to 480 °C. Vespel® SP-21 and SP-22 parts are favored for transmission manufacture because they are able to withstand pv loads (a product of pressure and speed) of up to 40 MPam/s in lubricated applications. “Of critical importance to ZF was the fact that Vespel® SP-21 thrust washers were able to take up the axial forces present in the transmission almost without abrasion from other adjacent metal components,” comments Peter Bergmann, market segment leader for the European transportation industry in the Vespel® business unit. “The exceptional tribological properties of the material lead to lower friction losses than with metal-to-metal pairings, and can thus have a positive influence on start/stop performance. It is for these reasons that Vespel® thrust washers have been successfully and reliably used for decades in transmissions for cars, buses or – as in the case of ZF – for agricultural machinery.”
All the Vespel® thrust washers used by ZF, have a small stud which locks into place with a corresponding recess in the cast-iron planet-wheel carrier. As a result, relative movement only occurs between the planet wheel and thrust washer. Moreover, as there is no relative movement between the washer and the carrier, there is no need for any additional finishing of the metal surface.
Jürgen Pohlenz, development manager for agricultural transmissions at ZF adds: “The Vespel® thrust washers adopt an excellent position with regard to their tribological behavior. Immediately after start-up of the transmission, they quickly adjust to any unevenness in the contact surface that may have been caused by play in the components or load tilts. As a result, after a minimal amount of running-in wear, a thrust surface is created, which is capable of evenly transmitting axial forces across its entire area. Friction is limited to very low levels during the lifetime of the transmission, meaning that the washers make only a minimal contribution to warming of components and oil.”
Vespel® rectangular cross-section rings control oil flow
As well as thrust washers, Vespel® SP-22 rectangular cross-section rings using microcut technology are used on the Eccom transmission models 3.5 and 4.5, which are designed for engines with an output of approximately 280 to 385 kW. They come with a precisely positioned, tightly-closed joint, which allows the ring to be attached precisely on the housing bore holes and to compensate for thermal expansion. The remaining gap permits the low and controlled flow-through of hot transmission oil. Oil grooves positioned on both sides in the seal face enable the oil film between the contact surfaces to quickly build up, helping to reduce friction, cool the sliding surfaces and extend the load range. With the grooves, the oil film remains stable even when oil temperatures reach 80 °C during operation and when pulsating pressure peaks of up to 22 bar occur, as is common for such transmissions.
Pohlenz continues: “Today’s tractors can reach speeds of more than 50 km/h (approximately 30 mph), associated with transmission shaft speeds of up to 4200 min-1. The relative velocities between the shaft and the rectangular ring are correspondingly high. Under a simultaneous pressure load, pv values of over 22 MPam/s can occur at the seal sites. In such cases, the tribological superiority of Vespel® rectangular cross-section rings comes to the fore when compared to standard modular graphite cast iron solutions. The equivalent metal rings can abrade due to the applied load, which in turn can lead to a thermal overloading of the shaft at the seal faces. In contrast, the low-friction, low-abrasion Vespel® cross-section rings continue to retain an optimal lubricating film even under such conditions. This helps effectively avoid unwanted overheating of the seal area, caused by friction, for the entire lifetime of the transmission.”
Tried and tested thousands of times
The ZF Eccom CVT transmissions have proven themselves more than 50,000 times in practice. The five current models, the Eccom 1.5, 1.8, 2.0, 3.5 and 4.5, are designed for engine outputs of between 100 HP and 520 HP (approx. 75 kW to 385 kW). The new Eccom 5.0 will be launched before the end of 2010, a transmission that is particularly well-suited for use in high-performance standard tractors, centre-pivot steering vehicles and agricultural towing vehicles, and is capable of transmitting engine outputs of up to 385 kW and input transmission torques of up to 2,300 Nm. All models enable a sensitive and uninterrupted build-up of traction on start-up thanks to their step-less ratio changes – even on the most difficult of ground conditions and without operating a clutch or manually shifting gears. The CVT transmissions consistently respond to load variations with gear ratio changes that ensure that output is always maintained at the best engine operating point in terms of consumption.
ZF is a leading worldwide automotive supplier for driveline and chassis technology with approximately 60,000 employees at 123 production locations in 27 countries. In 2009, ZF achieved a turnover of just under EUR 9.4 billion. The company ranks amongst the 10 largest automotive suppliers worldwide. To sustain its success with innovative products, ZF invests at least 5 percent of its annual sales in research and development. In 2009, a turnover share of 14% or EUR 14.3 billion was allocated to ZF products for agricultural and construction machinery, marine, airspace, specialty and rail vehicles.
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