Why does clutch plate burn and pressure plate crack easily?

Clutch plate burning and pressure plate cracking are two frequent catastrophic failures of dry clutch assemblies in commercial vehicles, tractors and construction machinery. The clutch plate suffers burning damage mainly from excessive frictional heat accumulation, while pressure plate cracking arises from combined thermal stress, mechanical fatigue and structural defects. These two failures often occur simultaneously and accelerate each other, stemming from improper driving, overloading, oil contamination, poor maintenance and defective assembly.

Long-term semi-clutch operation is the primary cause of burnt clutch plates. When drivers keep the clutch in half-linkage during uphill waiting, low-speed crawling or repeated frequent startups, the friction lining of the clutch plate maintains continuous relative sliding against the pressure plate’s working surface. Sliding friction converts massive mechanical energy into heat in a short time. Without sufficient cooling airflow inside the clutch housing, temperature rises sharply above the tolerance limit of organic friction materials. High temperature carbonizes the friction lining, produces a strong burnt odor, and even forms charred hard lumps on the plate surface. As burning progresses, the friction coefficient drops drastically, triggering severe clutch slipping that generates more heat in a vicious cycle. Heavy throttle during semi-clutch engagement aggravates thermal impact, causing local ablation and blistering of the clutch plate within a short mileage.

Overloading and uneven load distribution double thermal load and mechanical stress for both components. Vehicles carrying cargo beyond the rated capacity need larger clamping friction torque to transmit power. The extended sliding time during startup generates extra heat to scorch the clutch plate. Meanwhile, asymmetric weight creates unbalanced clamping force on the pressure plate. Partial over-compression concentrates stress on local diaphragm spring fingers and the pressure plate’s casting body. Repeated alternation of high temperature and heavy mechanical load produces thermal fatigue microcracks on the pressure plate surface, which gradually expand into penetrating cracks under long-term vibration. For vehicles towing overweight trailers, instantaneous torque shock easily tears already heat-weakened pressure plate structures.

Oil leakage contamination inside the clutch housing greatly speeds up clutch plate burning. When crankshaft rear oil seals or gearbox input shaft oil seals fail, engine oil or gear lubricant splashes onto friction surfaces. Oil forms a slippery isolation layer between the clutch plate and pressure plate, leading to persistent slipping even under normal driving conditions. Sustained sliding friction raises temperature rapidly and burns the friction lining thoroughly. The high-temperature oil vapor also corrodes the pressure plate’s metal surface, forming corrosion pits that act as stress concentration points. Under cyclic thermal expansion and contraction, these pits develop into crack sources on the pressure plate casting.

Damaged matching accessories indirectly induce dual failures. A failed release bearing cannot fully separate the pressure plate diaphragm springs, resulting in permanent micro-sliding friction that continuously overheats the clutch plate. Worn clutch fork bushings deliver uneven pushing force, making partial areas of the pressure plate bear excessive pressure and crack easily. Without functional shock absorbers on the clutch plate hub, road impact vibration transfers directly to the pressure plate, aggravating fatigue crack propagation. If pedal free travel is never adjusted, incomplete separation creates long-term hidden heat accumulation, worsening both burning and cracking risks.

Improper assembly and unauthorized modification destroy the original stress design of the pressure plate and accelerate cracking. Uneven cross torque when tightening pressure plate fixing bolts creates internal residual stress inside the casting. Random welding, cutting or local heating to repair deformed pressure plates destroys the original quenched metallographic structure, reducing material toughness and making the plate brittle under thermal cycling. Replacing only a burnt clutch disc while reusing a warped pressure plate creates uneven contact; concentrated pressure on small zones quickly generates new cracks on the pressure plate and re-burns the new clutch plate within a short time.

In conclusion, clutch plate burning is driven by sustained sliding friction and overheating, while pressure plate cracking results from superimposed thermal stress, mechanical fatigue and surface stress concentration points. Unstandardized driving and overloading are the root inducements of both faults. Timely maintenance and correct operation can effectively control heat generation and uniform stress distribution, avoiding burnt friction linings and cracked pressure plate castings.

References 

APA 7th Edition

Li, H., Wang, L., & Zhang, Y. (2019). Thermal wear analysis of automotive clutch pressure plate and friction disc under frequent start-stop conditions. Journal of Engineering Materials and Technology, 141(4), 041008.

MLA 9th Edition

Li, Hao, et al. "Thermal Wear Analysis of Automotive Clutch Pressure Plate and Friction Disc Under Frequent Start-Stop Conditions." Journal of Engineering Materials and Technology, vol. 141, no. 4, 2019, p. 041008, 

GB/T 7714-2015

[1] LI H, WANG L, ZHANG Y. Thermal wear analysis of automotive clutch pressure plate and friction disc under frequent start-stop conditions[J]. Journal of Engineering Materials and Technology, 2019, 141(4):041008.