Best Practice Clutch Fitting Advice From ZF Services UK
ZF Services UK supplies the automotive aftermarket with original equipment clutches and associated driveline components under ZF’s product brand Sachs. As with any product, a certain level of warranty returns can be expected despite stringent checks at every stage of manufacture and during quality assurance procedures. However, where Sachs clutch assemblies are concerned, investigation reveals that the problem is related more to fitting technique than to product quality.
Based on a review of recent warranty claims, Wayne McCluskey, technical training manager at ZF Services UK Ltd, offers some advice on adhering to best practice as prescribed by the manufacturer.
During the review, a pattern emerged of clutch assemblies regularly being returned with faults attributable to issues during workshop fitting or inspection, namely:
Misalignment of the gearbox during fitting
Excess and/or incorrect lubricant applied to the gearbox input shaft or clutch driven plate splines
Oil leaks from engines and gearboxes contaminating clutch friction linings
Distortion of the clutch cover
Distortion of the clutch driven plate
These problems are described in more detail below, together with suggestions for preventing them.
The gearbox input shaft should form a natural extension of the engine crankshaft, aligned on precisely the same axis. Where misalignment occurs this may be parallel, i.e. input shaft offset sideways/above/below the crankshaft axis but still parallel with it, or angular, e.g. gearbox tilted/skewed from the crankshaft axis (Figure 1). Any misalignment of the gearbox causes the clutch to be pulled out of line or damaged, reducing service life (Figure 2).
The following checklist will help to prevent or correct misalignment:
- All mating surfaces should be clean and clear of any debris
- Route all pipes, cables, etc., clear of the crankcase to gearbox joint a trapped earth strap or cable tie is sufficient to cause misalignment
- Any locating dowels on the engine/clutch bellhousing must be present and in good condition
- Assess condition of pilot bearing (where fitted) in the flywheel, and gearbox input shaft guide sleeves any damage can impair input shaft location, causing offset.
When installing the clutch driven plate, the correct lubrication procedure for the gearbox input shaft splines can be summed up as: a) use the correct lubricant, and b) apply it sparingly. Common faults are the application of excess lubricant, which is subsequently thrown out onto the friction linings causing clutch slip (Figures 3 & 4), and the use of unsuitable greases.
A special grease is supplied with Sachs clutch kits (also available separately from ZF Services UK), and should not be substituted. Copper-based grease is unsuitable; it has too low a melting point and is not formulated to repel clutch dust. As a result, it thickens in service, causing the clutch driven plate to stick on the gearbox input shaft, which gives rise to clutch judder.
After applying a small amount of grease to the input shaft splines, it should be distributed evenly by sliding the clutch driven plate back and forth a few times on the splines before removing any access.
Oil and fluid leaks
Aside from excess grease applied during installation, there are some other sources of friction lining contamination leading to premature clutch wear. With the gearbox removed, oil or fluid leaks at the crankshaft rear oil seal, gearbox input shaft seal and clutch hydraulic system can be noted and corrected. Cleaning the bellhousing thoroughly and removing any debris from a previous clutch failure will also help to guard against contamination of the new clutch.
Clutch cover distortion
Around 40 per cent of clutches returned as a result of disengagement problems have a distorted clutch cover housing. Using the correct fitting tools (Figure 5) can eliminate this problem. Sachs XTend self-adjusting clutches seem particularly susceptible to careless fitting – they are often returned adjusted fully outwards, indicating that they were installed without use of the appropriate tools. In this position there is no operating clearance, and the clutch cannot be reset.
Distortion of the clutch driven plate
Correct support of the gearbox during fitting is essential during reinstallation. Driven plate distortion is another high volume return, the maximum permitted on a Sachs driven plate being 0.5mm. Distortion of the plate in two positions 180 degrees apart indicates that the unsupported weight of the gearbox was suspended on the input shaft while engaged with the hub of the driven plate.
As vehicles continue to evolve in technical complexity, there is a clear need for technician training to develop and maintain best practice in the workshop. In response to this demand, ZF Services UK provides OEM-level technical training to the aftermarket (Figure 6) – visit www.zf.com/uk to find out more.
Wyatt International Ltd
Phone: +44 (0)121 454 8181