CMP brake question

[Lynn Eades]

Hi Phil, I could supply the bolt and snail, but unfortunately I don't have the spring as nos.
I could strip them from a back plate but they are rusty.
Surely someone closer can help, but if no one steps up, send you address.
Lynn.

[Phil Waterman]

Hi Lynn


Thanks for the offer, but I think I have a sources for a replacement springs that I can cut to length. Comes in 36 inch lengths. Another supplier looks to have ones which will work but has a $40 minimum order.



So I got to do a little more measuring, in particular is the spring force to compress the spring one inch.


Cheers Phil

[Grant Bowker]

I suggest caution cutting a spring for this application from a longer coil as the springs I have seen for the snail adjusters have flat, closed and ground ends. I believe most springs of similar dimensions (free length, wire diameter and number of turns) will behave similarly unless there is a large difference in the material properties. I have found several springs wanted for CMP projects at https://www.centuryspring.com/ You can search their catalog online by inputting whatever information you have about the spring you are trying to match.

[Phil Waterman]

Hi Grant



Century is one of the sources I was looking at this afternoon, spring compression strength I will try measure tomorrow.


Cheers Phil

[Phil Waterman]

Ok finished replacing the Brake Adjustment Cam Tensioner Springs.

Found two sources of springs that looked like the right size and spring tension, more on that later.

First step was cutting new adjuster bolts with the squared of area for the cam plate to lock into.

Cam 1.jpg Cam 2.jpg

I cut the flats by chucking the bolts up in lath head, and cutting the flats with High Speed Grinder attached to the tool head. This could be done with a flat file and vice. Either way you want to get as tight a fit as possible.
The bolts are Grade 8 1/2 bolts, found that course thread work better than fine thread.

Cam 3.jpg

Once the bolts are notched down the correct length for the spring with a spring washer and cam plate you can check the torque required to turn the bolt head

Cam 4.jpg

[Phil Waterman]

One of the things discovered was that only one washer should be used as using more than one spring reduces the torque needed to turn the adjuster.

I tested two different types of spring from two different suppliers and found that they both yielded about the same results with the adjuster bolts when installed and the springs compressed short of being coil bound or totally tight of 10-15 Ft.LBs. of turning torque (initial torque to get the adjuster moving was around 20 Ft.LBs)

Cam 6.jpg

Do not paint or grease the contact sides of the cam plate or backing plate as this greatly reduces the force to turn the adjuster. Which was the problem with the broken springs.

Now how to attach new bolt to cam plate while holding tension on the spring, I when simple put a nut on the bolt thread tighten it down, but with a spacer that would allow me to weld the bolt to the cam plate. Then with Mig welder turned up welded one side of the bolt and then the other.

Cam 5.jpg Cam 7.jpg

Then just cut off the excess to make clearance on the back side of the brake shoe and it is done.

Cam 8.jpg Cam 9.jpg

The adjustments now take a nice steady force to turn so I expect that the backing off of the adjusters will be a thing of the past. Yes, I’m going to give both adjuster a horizontal witness line so that any movement will be easy to spot.

There is only one bad thing about crawling around under the truck and having the wheels off the ground, you spot other things that need attention. See Loose Universal Ends.


Loose Universal Ends