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Brake Caliper falling apart

Peter wrote:

There is a big difference between a dry thread and a lubed thread. The required torque is different.

The difference may indeed be huge. Many years ago, I was swapping the cylinder head in a 1975 model Volvo 164. The head bolts were to be torqued dry, and I, lacking experience, lubed them thoroughly. One bolt snapped in the course of torquing and one more took a hourglass shape.

Loctite does also act as lubricant, though not as well as oil.

LKBU (near Prague), Czech Republic

It does make me wonder how inaccurate the actual tension on the bolt must be, given the big variations in friction between even dry surfaces. And imagine someone assembling parts with greasy fingers; they will get some of it on the thread and then the bolt will get much more tension.

That’s why conrod bolts are tightened with a micrometer which measures the actual bolt extension, AFAIK.

One can easily strip the thread on a high tensile 1/4" UNC or UNF bolt, with a normal spanner, about 15cm long. I have seen it. A torque wrench is really essential.

I also don’t like the increasing tendency to use shakeproof washers. Surely they affect (reduce) the bolt tension for a given torque? But they are cheap which is why they are found everywhere… they just bugger up the surfaces they face. Spring washers are much nicer. I saw a Lyco engine the other day which was done entirely with shakeproof washers… really tacky IMHO.

Administrator
Shoreham EGKA, United Kingdom

For my (relatively new) Cleveland brakes, the dry bolt torque is marked on the caliper ID plate and in the manual. After 40 years the markings on the caliper may not be legible, and as Peter says, some technicians may be better at using a torque wrench than others..

Glenswinton, SW Scotland, United Kingdom

There is yet another technology to lock the threads while maintaining the right tension – green (wicking) Loctite 290. Unlike the regular purple, blue and red grades, it is much thinner, applied after assembly, and penetrates the engaged threads.

LKBU (near Prague), Czech Republic

Sounds like over the years they were over torqued too many times and failed. It does happen. Don’t beat yourself up about this, things will and do fail, sometimes without explanation. Every annual/100 hr inspection I would make sure the calipers studs and retaining plate are cleaned thoroughly. If the studs of your caliper do not move freely when pressure is applied, the bolts holding your pads together will be under abnormal tension.

For a brake caliper and given the fact you maintain your aircraft thoroughly, Loctite is way overkill. For piece of mind, buy a set of AN bolts with safety holes drilled in the head and safety them together. If not standard torque for those bolts is more than adequate.

The Loctite I have used is blue (243). It comes in a red bottle, hence my confusion. I understand the red stuff is stronger and the green stuff criminal tight.
Simon

Thanks for all of the replies. My maintenance organisation were very surprised about this. They routinely throw bolts away which have signs of thread wear & they are diligent with the torque wrench.

I’ve yet to read the manual for torques; 90 foot lbs is mentioned above seems huge for that tiny bolt!

What do people think about using copper grease to lube the threads and I wonder what an appropriate torque would be? If copper grease was used then perhaps the bolts would need wire locking?

United Kingdom

I have Loctite 278 in the workshop. Red bottle, the stuff is green, and it is high-temp. Never used it on the plane however. Wire locking is much better, and drilled bolts cost only very slightly more.

90 foot lbs is mentioned above seems huge for that tiny bolt!

It is 90 inch-pounds. 7.5 foot-pounds. It’s a pretty standard torque for 1/4" bolts in aviation, which are normally “high tensile”. The max is about 100 and that is e.g. the value for the bolts which hold the crankcase halves together.

Administrator
Shoreham EGKA, United Kingdom

That would explain it! 7.5 foot pounds sounds better – I was being a bit thick there!

United Kingdom

The “wet or dry torque” discussion continues here

Administrator
Shoreham EGKA, United Kingdom
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