I am having trouble understanding the front brakes on my 140 . Could someone enlighten me please ?
Thanks Jim
Jim, the ratchets are there to take up wear, so that each shoe only moves the minimum distance necessary to contact the drum. Over time the ratchet ensures that the base position of the shoe moves up the ratchet, to compensate as the lining wears. The shoes don’t need to be pulled strongly back to the ‘off’ position, they just need to be out of contact with the drum. Think of disc pads, no springs at all - they just rest gently on the disc. That said, I’ve not got into my brakes yet but maybe it’s worth checking that you have the backplates assembled correctly, and on the correct sides. I’d have expected the spring to be acting on the leading edge of the shoe rather than the trailing edge, but someone who knows will advise you on that.
Jim,
The two shoes should both have 2 springs attached to each shoe in order to keep them firmly against the brake pistons. So the other spring is not attached to the back plate (as I heard you say). The only purpose of the “ratchet” mechanism is to automatically compensate for wear of the lining, meaning as soon as the “slack” is becoming larger than the distance between two teeth of the ratchet, the distance is compensated.
Bob K.
I would expect that both shoes have 2 springs attached . But look at this spring from David Manners , one hook is at 12 o’clock the other at 3 , shouldn’t they point in the same direction ?
.
What confused me regarding the ratchet is the teeth are very small allowing for a minute amount of slack .
Jim,
You’re correct: I confused front and rear brakes. So the front brakes have the springs at one end to the shoe and at the other end to the pin on the back plate.
Bob K.
I think you might be over thinking this James. you have the springs installed correctly and I have attached photos of my original backplates. (one has never been disturbed) there is float in the interconnecting links that allow the ratchet to work as described above. have you gently operated the brakes and watched what’s going on? (needs an assistant, and a couple of screwdrivers to replace the brake drum)IMG_2488|375x500
You are experiencing a very common misunderstanding of how the self-adjusting system works…as are all who have responded so far. You MUST read the “Principles of Operation” section of the manual, which makes clear, though tersely, that the ratchet/serrated bar/spring clip assembly is NOT the adjustment mechanism. That ratcheting system is merely an emergency back-up arrangement in case the friction pads lose their friction. The other end of the serrated bar is where the shoe-to-drum clearance is maintained.
I have written quite a bit about this before so feel free to search the archives.
Got it ! Thanks for your help .
Lee,
The “self-adjusting” mechanism of the (later) Lockheed XK 120 and XK 140 is required to compensate for lining wear. This is a different mechanism than securing clearance between lining and drum via the enlarged hole of the (other end of the) adjustment bars.
If you have new shoes installed, you will have to pump the brake pedal several times and it is normal to see adjuster bars move one or even two ratchet teeth.
We agree that the ratchets are also a safety device in case the friction pads no longer work. But if you continue to drive over a longer period with these “malfunctioning friction pads” you will end up with extensive pedal travel as the wear of the lining is no longer compensated.
Bob K.
The friction pad device is to allow the serrated bar to effectively lengthen as the shoe surfaces wear, meaning the pads must resist return spring pressure (let’s say 20 lbs), but not have so much grip as to create excessive pedal pressure. This is why the pad’s bolt is spring-loaded.
Bob:
A question, what does one use as material to replace the “friction pads”? While I am aware that they are available from the usual sources I am curious to know exactly what they are made of. Back when I first purchased my 120 and refurbished the brakes the shop who turned the drums and lined the shoes made up pads using what appeared to be some thin type of lining material.
Chris.
Chris,
What I remember of the replacement pads I ordered some years ago, they were a woven friction material similar to other woven linings, but of course much thinner. But I’ve also seen “non-woven” versions (see photo).
Where standard (brake) lining materials start around 3/16" (4.8 mm) thickness or even higher, these pads are much thinner. There must be companies that manufacture these kind of materials on the role or as sheet.
Bob
