Prop bolts....what's the consensus?

[jon595]

I never used a torque wrench to tighten my prop bolts either. That is, until I had two engines twist crankshafts. In talking with the service person, he stated that improperly tightened prop bolts will cause the vibration that allows it to twist.

 

[ghoffman]

The actual general rule in engineering is the engagement should be equal to or greater than one times the dia. of the fastener. But the common sense rule of thumb is two times the dia. for fasteners under load. In our case we have load and vibration so I would never fly with 1/4" of threads in the hub.

Agreed, that is pretty much a good rule of thumb in aluminum. But it can be calculated, there are curves that give you the shear area of the threads per length of engagement. For example, a M5 (nut) is very close to a 10-32 in diameter, so lets use the 10-32 as an example. A 10-32 has about .3 square inches of shear area per inch of engagement. So, 2 diameters would be 0.4 inches long, times 0.3 square inches/inch or .12 square inches of shear area. A 10-32 torqued to 35 inch-pounds has 1000 pounds of load (or 2000 lbs at 70 in-lb torque, etc). So, 1000 lbs/.12 square inches equals 8300 PSI and most aluminum castings yield at ~30,ooo psi. BTW, DA uses 7075 billet aluminum which is more than double that strength. The more engagement the better up until the point that the fastener just fails in tension..

 

[BalsaDust]

So quick question to those on this thread. I forgot to measure a bolt before leaving for work and want to order a box of bolts for my DLE 55. I know they are M5 bolts but how long are they supposed to me? Would like to order them tonight.

 

[Snoopy1]

I agree with all that has been said but I do know that not all the threads that are in gaged are carrying the load. From experience and in depth testing in steel mills, must state that all this was done with very large bolts and with loads that were several thousand tons. It was found with different types of threads ie rolled or cut by a machine that we could never get more than three full threads to in gage at a time before we got deformation of some kind in the threads when load deformation takes place, and I belive that we need to be very careful on how we tighten the bolts one propellers. Please take care when tightening bolts on a propeller. Once it is done properly it should not be retightened over and over if you are then you are crushing the the propeller or elongating the the bolts, something will fail soon. Tighten the bolts with in the elastic range of the propeller, motor hub or bolts. When done correctly it will stay tight and not need to be retightened.

 

[ghoffman]

Yes, typically the first thread carries about 40% of the load and by the time you get to the 5th thread, it probably is not doing much.

 

[yakken]

I like to use nylon prop bolts. [emoji15]

 

[flycanic]

I just run out to the local Cat House and get the bolts I need. I never use lock washers. I just order (flat) hard washers from CAT and if they do not , have them in stock they can have them over night to any Cat House in the Country and there is no shipping charges Cat has trucks running parts to all there dealers 24/7. What the heck is a NERD LOCK ? As far as thread depth I like to have the bolt just show on the back side of the prop hub. I might change the bolts once a year. I use my torque elbow when tightening in a star pattern. As a mechanic for years I have turned millions of bolts and after years of turning bolts for a living you develop a feel for what torque is needed for all sizes. I never install the bolts DRY installing a DRY bolt in the aluminum threaded hub will give you a false torque reading. By putting a drop of oil on the bolts will give you a tighter holding strength and a more even holding strength on all the bolts. That's my 2 cents worth.

 

[BalsaDust]

Nord lock is a different type of lock washer that works really good.

 

[thurmma]

I use the Nord lock on muffler bolts and prop bolts. Haven't had one back out yet that was properly tightened

 

[dhal22]

Does anybody use thread lock on prop bolts?