A common clutch failure issue that is often misdiagnosed is chatter. Typically, chatter may develop 30 or more days after clutch installation and after the clutch left the service center in good condition. In most cases of chatter, the affected clutches have been installed and serviced the same way that they have been successfully serviced for many years. Why, then, all the chatter?
The clutch is part of an overall system and sometimes it's critical to look further than the clutch for the source of the chatter. It may not always be the clutch, but in many instances, the flywheel that is the cause. (Fig. 15)
The use of resurfaced or remanufactured flywheels, which often still have hot spots that cannot be removed during resurfacing, will cause chatter in a majority of cases.
A recent study conducted by a leading clutch manufacturer on a Dodge pickup with a 5-speed manual transmission emulated what a technician might go through when installing a new clutch set and a resurfaced flywheel.
The flywheel was taken to a local machine shop for resurfacing. The charge for resurfacing was $30 and it was ready the next day.
Prior to sending the flywheel out for resurfacing, the hardness values were checked using a NuLine TH134 hardness tester. By marking the flywheel, then measuring the coordinates, a map was created of the actual measuring positions. After resurfacing, the same exact spots were checked to see what the hardness levels were after grinding.
The baseline hardness levels of these flywheels ran about 250-260HB (Brinell) and the hot spots measured in the 500-600HB range. After resurfacing, five of the high value spots actually tested harder and only one hot spot tested lower. The hot spots were still present on the flywheel. They only looked better. (Fig. 16)
The effect hot spots have on friction material is what causes chatter. The HB hardness values are just that, values. To put them into common terms, typical stainless steel is about 250HB, glass is about 550HB and hardened tool bits are about 650HB. A regular drill bit will drill into the edge of the a typical flywheel, but try to drill into the hot spots with the same drilling pressure and the bit will just sit on top of the surface and heat up.
The hot spots on the friction material causes a series of changing wear rates due to the surface hardness variations and the actual thickness variation of the flywheel. In severe cases, the actual surface of the flywheel gets hot enough to become like melted plastic and smear across the surface.
The best way to guard against chatter and hot spots is to install a new flywheel at the time of the clutch installation.
- Thanks to Perfection Clutch
Leave a comment