clutch bolts sheared

[vtjeep]

Ok how many of you had this happen and why dose it what did i do wrong ? i was trying to hover and touched down then engine died so went to restart then binded so i was confused so dropped engine no clutch on fan !! it broke off and sheared the bolts what the hell!! should i buy the 46$ fan or just get the stock one to replace ?

[capebob]

This happens when the bolts bottom on the fan before tightening the fan. You probably use a bolt a mm or two long. The stock fan is fine just make sure the clutch bolts aren't too long and use thread lock.

Bob

[vtjeep]

ok so the bolts the kit came with were wrong ? ok im ordering new fan an clutch this time try to c if there to long and cut them shorter ..

[capebob]

I've built many Raptors over the years mostly for other pilots and I've found your problem in two or three machines. I think it's caused by some of the fans not being threaded deep enough. If you get a new fan it may be OK if not you can either shorten the 10mm screws or use 8mm screws. Of course your problem may have been caused by either no thread lock or the screws weren't torqued down tight enough. These must be tight.

Bob

[vandelescrow]

I've had this happen to my bird using the aluminum fan. The fan has 4 bolt holes so after it happened the first time, I got new bolts (size from the manual) and put them in the other set of holes, those bolts broke also, fan no longer any good. Locktight was used, tightened as tight as I could get them, yes the shim was placed under the clutch.

After reading the above posts it now makes sence why this happened, if the fan was not machined properly.

[archiebald]

I never heard of or seen this before - must be either;

1) incorrect assembly. (and by that I also include failure to check on bottoming screws)
2) crappy screws.

Vandelscrow,
I've had this happen to my bird using the aluminum fan. The fan has 4 bolt holes so after it happened the first time, I got new bolts (size from the manual) and put them in the other set of holes, those bolts broke also, fan no longer any good. Locktight was used, tightened as tight as I could get them, yes the shim was placed under the clutch.

I would guess that the part I highlighted is more likely to be your problem.

[BarracudaHockey]

This isn't all that uncommon and is usually like Bob said, the fan isn't threaded deep enough and it looks like its tight but there is some play, or the bolts are over tightened and break.

[alexander]

As uncommon as it is I'd think a small washer or two should take up the space.The manual shows the correct length bolt should you wish to verify this item in the remaining two holes. Places that a similar bolt might go are washout, head mixers, main gear and aft gearbox.

Stephen

[Laurens]

I neved heard of the bolts being too long, but the cases I've seen are all due to the bolts being overtightened.

[mazakari]

I've had this happen too - on the second tank of fuel on a bright warm summers day. It wouldn't have been so bad but the only other heli I brought was a TRex and that had kissed the ground no more than 30 minutes earlier.
I pulled the motor out to try and pick up the other two holes to mount the clutch, but they had sheared bolts still in them !!! I didn't have any easy-outs so that was the heli's finished for the day.
What I did do when I got home though - I drilled out the fan to take M4 bolts. I have never had a problem since. Maybe it's me being tight with my money, but it's a good cheap fix and a lot stronger too.

maz

[vtjeep]

i think i did over tighten .. we had the starter backwards to start so that backed off the main fan center nut when we put it back together i may have over tighten .. but when i took it apart the bolts were broke about 4 mm in the fan not sheared off at the top so it looks like bolts failed and i did use lock tight by the way..

[archiebald]

Many people are not aware that bolts and screws come in different grades. For metric screws under M4, these are typically 4.8, 9.8, and 12.9. The higher the number, the higher the UTS. (Ultimate Tensile Strength)

4.8 = 420 MPa
9.8 = 900 MPa
12.9 = 1220 MPa

Obviously, it is best to make sure that you are using the strongest of these.

In all cases, a major cause of bolts breaking is being overtorqued during assembly. Recommended torque values obviously vary for the different grades. Sorry, I don't have the values to hand for M3 machine screws.

Because these bolts are being screwed into the aluminium part of the fan, they need to have the longest possible thread engagement otherwise they will start pulling the thread during assembly which will then lead to loosening during operation.

Best Advice - Don't overtighten - especially not necessary if you are using loctite.