2.4 Release Bearings, Cross-Shafts, Clutch Safety Switches, Input Shaft Wear, and Clutch Torque Ratings

2.4 Release Bearings, Cross-Shafts, Clutch Safety Switches, Input Shaft Wear, and Clutch Torque Ratings

The ASE T3 task list includes several clutch tasks that are easy to overlook because each is small: servicing the release (throw-out) bearing, sleeve, bushings, springs, housing, levers, fork (yoke), pads, rollers, shafts and seals; checking pedal height and travel and the clutch safety switch; inspecting the input shaft and bearing retainer; and determining the proper clutch torque rating. Together they explain many "new clutch, same problem" comebacks.

1. The Pull-Type Release Assembly

On a pull-type clutch the release bearing is part of the clutch assembly. The bearing housing is attached to a release sleeve that slides on the transmission input shaft on a sleeve bushing. The transmission's release yoke (fork) is clamped or splined to a cross-shaft that turns in two bushings in the clutch housing, and the yoke fingers bear against wear pads on the bearing housing to pull it rearward.

Eaton's transmission-side checklist is specific. Excessive wear at the cross-shaft and bushings can cause side loading on the sleeve bushing, bushing failures, and yoke bridge contact with the clutch when the pedal is down. Worn yoke fingers can cause bushing wear and yoke interference. Eaton's overhaul inspection adds: replace yokes worn at the cam surfaces and bearing carriers worn at the contact pads, and replace pedal shafts worn at the bushing surfaces.

Lubrication (Eaton)

• Use a lithium complex grease with a minimum 325°F (163°C) operating range, meeting NLGI grade 2 or 3.
• Grease the release bearing through its fitting until grease visibly exits and contacts the transmission shaft. That grease also lubricates the clutch brake and the sleeve bushing when the pedal is pressed.
• Grease the cross-shaft bushings and linkage pivot points.
• Never add grease or anti-seize to the input shaft splines. The discs must be free to move, and grease there can be thrown onto the facings.
• Eaton warns that failure to properly lubricate the bearing and bushing will result in bearing and sleeve failures. On UltraShift PLUS ECA clutches, an optional "GI" reminder in the gear display prompts greasing.

Riding the pedal is the classic release-bearing killer. Eaton notes that it puts a constant thrust load on the release bearing, which thins out the lubricant and wears the pads.

2. Measuring and Adjusting Release Bearing Position

Release bearing position is the gap between the bearing housing and the clutch brake with the pedal up:

• 0.500–0.560 in. on Eaton manual-adjust clutches.
• 0.490–0.560 in. on Eaton Solo self-adjusting clutches.

It is set inside the clutch — with the adjusting nut or lockstrap on manual clutches, or automatically on self-adjusting ones — and never with the external linkage. Bearing position must be correct before the 1/8 in. yoke clearance and clutch brake squeeze are set (sections 2.3 and 3.1).

3. Pedal Height, Travel, and the Clutch Safety Switch

• Pedal height is set by the upper pedal stop. Eaton's guidance is that if more free play is needed, adjust the upper pedal stop to raise or lower the pedal. If that is not possible, check the OEM parts book to confirm the correct clutch arm was installed at the factory. The linkage must still provide Eaton's minimum 0.685 in. of yoke finger travel.
• Total travel must allow full release plus clutch brake squeeze. A thick floor mat, a bent pedal, or worn pivots can steal travel and cause drag.
• Clutch safety (start interlock) switch: Most trucks with a clutch pedal have a pedal switch that lets the starter crank only with the pedal pressed. On many engines the same or a second switch tells the engine controller when the clutch is released, for cruise and idle functions. Test it with a meter at pedal-up and pedal-down, and adjust or replace it per the OEM. Treat "cranks with the pedal up" as a safety defect; a misadjusted switch can also cause intermittent no-crank complaints.

4. Input Shaft and Bearing Retainer (Where the Clutch Brake Rides)

Before a new clutch goes in, Eaton has the technician check the transmission:

1. Transmission bearing retainer cap: a worn or rough cap may make the new clutch brake wear prematurely.
2. Input shaft length: Eaton's specification is 8.657 in. nominal and not greater than 8.71 in. (referencing the SAE handbook). If the length is greater than 8.71 in., replace the transmission bearing retainer cap.
3. Input shaft surface: wear or roughness where the sleeve bushing rides reduces bushing life and can let the bushing come out.
4. Input shaft splines: any wear keeps the discs from sliding freely, causing poor release (drag). Slide each disc the full length of the shaft to check for twisted splines.
5. Input shaft spigot (pilot): wear prevents a proper fit in the pilot bearing's inner race and can damage the bearing or the clutch.
6. Clutch brake: Eaton's instruction is simply "replace" when a clutch is installed.

Eaton's transmission overhaul inspection also says to check the oil return threads on the input shaft; if their return action has been destroyed, replace the input shaft, because oil can then work forward toward the clutch.

5. Determining the Proper Clutch Torque Rating

Clutch torque capacity depends on clamp load (N), coefficient of friction (μ), mean radius (r), and the number of friction surfaces. For its clutch sizes, Eaton simplifies the mean radius to r = (ID + OD) ÷ 4.

Illustrative example (not a specification): μ = 0.4, clamp load 3,000 lb, facing OD 15.5 in. and ID 10 in. gives r = (10 + 15.5) ÷ 4 = 6.375 in. (0.531 ft). One friction surface carries 0.4 × 3,000 × 0.531 ≈ 637 lb-ft. A two-plate clutch has four friction surfaces, so it carries about 2,550 lb-ft — twice a single-plate clutch with its two surfaces.

Practical rules from Eaton:

• Slippage is the result of loss of clamp load from lack of adjustment, loading the clutch beyond its design torque rating, or the clutch reaching the end of its design life.
• Because an electronically controlled diesel engine can be re-rated easily, confirm that the clutch and other drivetrain components can carry the increased torque and power before an engine re-rating is performed.
• Organic facings are limited to 1,400 lb-ft in 15.5 in. clutches, and Eaton recommends soft-rate dampers for vehicles over 1,000 lb-ft.
• Clutch life also depends on the application (torque capacity), gross combination weight, the number of starts per day, and maintenance and adjustment.

Match any replacement clutch to the engine's current torque rating, the transmission input spline, and the flywheel type, using the OEM's or clutch maker's application data.