Saturday, November 28, 2015
Air Brake Job Using Machining Tools
Machining School Bus Brake Shoes from John Whelan on Vimeo.
The lathe in the video makes a complete cut on the brake shoe allowing 100% contact. This is necessary when a chatter or brake pulsation is occurring with brakes applied. We used to machine all of our brake shoes on all brake jobs but found it to be time consuming. Now we replace the drum and shoes plus hardware.
There have been some cases where the contact is not favorable and machining was necessary. Our buses use all sizes of brake shoes from 6 inch on the steering axle to 8.5 inches on the drive axle. All air brakes come standard with automatic slack adjusters. I talked to a sales rep from Canadian Brake and he says there are still customers who purchase manual slack adjusters. I haven't seen one of those for years.
Since we run a fleet of school buses it's policy to replace the auto slacks every brake job. There is only one drive axle so 200.00 for a couple of slacks is nothing. Spring brake pots get replace every 5 years. Our choice is to use MGM piggy back pots because of their superior quality. For an extra 50.00 that's great insurance.
A few months ago I attended a trade show in town and talked to the Haldex slack rep and he informed me to not grease them too often. This will disable the mechanism with the hydraulic effects of the grease. We have auto grease systems installed on our S cam tubes and slack adjusters. That has caused grease leakage on to the brake shoes from over greasing which is picked up by the government vehicle inspectors.
If it's bad enough the wheel has to be removed and shoes cleaned up. This is a procedure we want to avoid so now we advise the auto grease system installers to avoid hooking up to those areas including the steering box bearing. That's another spot the DOT makes notes on during their fleet inspection thinking it's a seal leak.
Getting back to the brake lathe it does wonders on pesky brake problems that the operator experiences. There was one time when we just kept machining down the shoe material until there was a total 100% contact to fix a pulsation. The drums had to be machined as well. There was a lot of material removed to accomplish this but the front brakes last twice as long as the drive axle brake so it was worth the labor to do so.