Clutch Maintenance On Pro Saws
Most pro saw users in the Northwestern US will experience a problem or two with the clutches on their pro saws at some point during their saw's life. Much of this can be attributed to long bars and chains fitted on saws used in the Northwest. These long cutting attachments require more clutch "slip." Over time this causes extra wear on the friction surfaces of the clutch shoes and drum.
Other operating conditions also contribute to clutch wear. Running saw chain with depth gauges cut too low is not only dangerous, but can damage your saw's engine and clutch. Aggressive running techniques are yet another contributor to premature clutch failure. Understanding how operating conditions contribute to clutch failure will help you reduce or eliminate these factors.
While the use of long bars and other operating conditions may accelerate clutch wear, clutches on today's saws deliver good service life if they are maintained. Regular inspection and replacement of worn parts usually leads to trouble-free service. Unfortunately, clutches are a component that often doesn't get maintenance until it fails.
A clutch that is not maintained can fail and damage other parts. The parts above were replaced during a recent repair in our shop. While many of the parts needed to be replaced due to wear, parts like the oil pump were damaged by the failure and increased the cost of the repair.
Broken Clutch Springs
Have you ever had trouble with a saw that regularly breaks clutch springs? Almost everyone has at one time or another. Have you ever wondered why the manufacturer can't improve this weak part? While you are cussing the darn springs, more times than not, better quality springs is not the solution. The problem is actually caused by wear in the clutch drum and shoes. Broken springs are just a consequence.
Here's what happens. When the clutch wears, the surface of the shoes wear. The inside of the clutch drum also wears. After a while, for the clutch to engage, the shoes must travel much farther to contact the surface of the drum. This extra travel stretches springs beyond their limits. Sometimes the holes in the shoes where the springs are attached also wear. The spring tails actually start to sink into the shoe material. In both cases, the clutch seizes when the springs break, but the problem is really caused by other worn parts.
When inspecting a clutch, check for wear described above. Also check where the shoes slide on the spider and any guides if the clutch uses them. When you find excess wear on the shoes, springs and spider consider replacing the whole clutch assembly. Have a look at the clutch drum, too. If you see wear below the surface where the shoes contact the drum, this part should also be changed.
This clutch shows a lot of wear. The springs have worn slots into their mounts on the shoes. This clutch needs to be replaced. Newer style clutches have an improved design which reduces clutch shoe "shimmy" and extends the life of the mounting positions.
The tops of the shoes, where they contact the drum shows a lot of wear. Notice that these surfaces are thinner on the ends, indicating the shoes do not contact the drum evenly. This is normal, to some degree, because of engine rotation.
Clutch Failure & Causes
The following photos were all taken from repairs we performed in our shop. In these examples you will see the result of inadequate maintenance and/or poor operating techniques.
When the shoes slip inside the clutch bell, they wear its surface. With use, the bell becomes thin and weak. Eventually it cracks and fails to contain the shoes. This is the result.
In this photo, the center clutch shoe guide is cracked. This is often due to chatter. This occurs most often on saws whose operators run low depth gauges and/or run the saw aggressively.
On this clutch, the springs have been removed to show the damage to the guide portion of the top shoe. Like the photo above, this damage can be caused by chatter that occurs when running a saw chain with depth gauges that are too low.
On this clutch, the retaining clip has broken and is missing on the top shoe. Like the other examples above, chatter and hard use can cause these clips to flex and break.
This is a new clutch from a STIHL 460. Notice how thick the top surfaces of the clutch shoes are as compared to the examples above. Also notice the improved design. The clutch shoes "nest" with each other adding stability to the system. This new design also reduces "shimmy," which increases the life of the clutch springs.
Lubricating The Clutch Bearing
Another thing many pro users forget to do is lube the clutch bearing. This small needle bearing is mounted between the clutch hub and crankshaft. When you are cutting with the saw, the bearing does not turn. When you idle the motor, it is what allows the chain to stop while the engine still turns. On most models, it gets a little lubrication from bar and chain oil that drips down from the sprocket, but this is not enough. For this bearing to give a long service life, it needs occasional grease applied to it in the shop or by the operator.
While the bearing itself is not an expensive part, nor is it expensive to replace, it runs on the end of the crankshaft. If this bearing is allowed to fail, it may damage the surface on the crankshaft. Once this surface is damaged, even a new bearing won't last, so the crank has to be replaced. This is an expensive repair and can be completely avoided with a little grease and some routine maintenance.
Notice the galled inner surface of the hub where the bearing resides. When a clutch bearing is not lubricated and fails it can damage this surface. Unfortunately it often does the same to the outer surface of the crankshaft. When servicing the clutch bearing, check this hub. If galling is found replace both the bearing and hub. Never put a new bearing in a hub like this.
This information should help you get maximum performance from the clutch on your pro saw. Keeping the clutch parts fresh and some simple routine service, will eliminate down time and the frustration that goes with it. If you need information or have questions about a clutch problem you are experiencing on your pro saw, please contact us.
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