Bolted connection question

[mschro]

I have an application where a flanged shaft is bolted to the rotating flange on a hydraulic motor. The bolts being used are gr12.9 - M24's and torqued to 664 ft-lbs (lubricated with loctite). The motor is threaded, so the bolt with a narrow washer (against the bolt head) goes through the flanged shaft and into the threads of the motor. The material of the flanged shaft is 4140 with a RC of approx 30.

Unfortunately, one of our engineers designed the through holes on the flanged shaft to accommodate one inch bolts instead of the M24's. The through holes are 1-1/16", thereby creating an overall clearance of .118" (instead of our standard .063").

The question is: Is this too much clearance and what adverse effects will it have? The flange shaft is piloted to the motor, so alignment shouldn't be a problem. I have some ideas, but I wanted to get some additional feedback.

The motor is generating around 120,000 ft-lbs of torque, so obviously I want to be very cautious about this connection.

Thanks

 

[desertfox]

Hi mschro

Coarse clearance hole for M24 is 28 dia therefore the 1 1/16" shouldn't be an issue.
The issue's for me are:-

1. Are you just allowing the bolts to take all the shear
stress generated by the torque or is there another
part to take the shear load?

2. You haven't specified the radius of the bolt circle so
I don't know if you have suffient clamping force to
prevent relative movement of the flange faces or what
clamping force your expecting, however the method you
are using ie torque setting is subject to +/- 25% error

Referring to my first concern with clearence holes and bolts
you cannot be certain how many bolts are in contact with the hole face and therefore how many bolts are actually taking the shear load.
Finally have you made an error with the torque setting figure as the recommende torque for an M24 12.9 is 1144.2Nm
which equates to about 842 lbf ft.

regards

desertfox

 

[desertfox]

Hi again

My apologies about torque setting I read the 120000lbf-ft
as the bolt torque setting.

regards

desertfox

 

[CoryPad]

The major effect of the hole diameter is on surface pressure of the clamped components. As long as there is sufficient surface area in the contact areas of the clamped parts, you shouldn't have a problem.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[dimjim]

I would be a little concerned about the bearing pressure
under the washer heads. You might need heavy washers
if it is a problem. The torque values do look a little
low. I assume you got those values from the manufacturer of loctite. Did they know you were using grade 12.9 bolts?

 

[mschro]

Thanks to everyone for their feedback.

dimjim (and others with an opinion),

I was a little concerned about the bearing pressure under the washer as well. I'm not sure how to best analyze this. Do you have any recommendations?

My initial thought when presented with the situation was to stack two washers under each bolt head. Would you agree that this would be an easy solution as well?

 

[CoryPad]

Surface pressure won't be affected much by stacking washers - you need to control the inner contact diameter (the hole) and the outer contact diameter (washer outer diameter) to provide enough surface area to avoid embedment.

The calculation is straightforward. You define the maximum fastener force expected (combined pretension plus service forces), then divide that by the allowable surface pressure of the joint material(s). This number is the required surface area. If you have a fixed inner hole, then calculate the outer diameter you need to achieve this surface area.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[mschro]

Thanks corypad,

The thought with the double washer was to add stiffness and avoid (reduce) any deflection of the washer above a oversized hole.

 

[desertfox]

Hi mschro

I would use a heavier washer as suggested by dimjim if
your concerned about washer deflection.
Introducing another washer may affect the under head friction during tightening ie:- another mating face which
may have relative movement.


Regards

desertfox

 

[byrdj]

You might want to look at "belleville washers", they will help with negating washer deflection.

 

[CoryPad]

Using two washers (or a belleville washer, somtimes referred to as a conical spring washer) certainly will reduce the washer deflection over a clearance hole. In your case, it doesn't seem like deflection will be a problem, but you haven't given enough information to be certain, so only you can make that call.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[IceStationZebra]

Belleville's have there place, but I don't think this is it.

-If the belleville is partially compressed your clamp load is determined by the spring rate. And by default this is "deflecting" and you will only have partial surface contact. And if there is a significant axial or overhung load you may have to worry about the washers fatiguing.

-The other situation is that the belleville is so stiff it will not deflect or it is so soft it flattens out, in which case you should just use an extra thick hardened washer and be done with it.

Two situations (and I'm sure someone will come up with a third and fourth) were belleville washers work in clamping situations:
1)there is insufficient fastener length to provide the necessary stretch and therefore the necessary preload over the range of use (often times different thermal expansion rates). But by the time the belleville comes into play the clamp load will have decreased significantly and obviously not all applications can handle this.
2) when the preload required is lower than what will stretch the fastener. In this case the washer will be partially collapsed, but you must watch for fatigue.

I have been involved with about a dozen varied situations where the lowly belleville was to be the savior but it never worked out that way. ISZ

 

[Tmoose]

Bolted joints that rely on friction can be quite reliable.

The USArmy claims their top fuel dragster accelerates at 6 gs.

http://www.goarmy.com/racing/nhra_top_fuel_dragster.jsp

I'm not sure how many lb-ft of torque that requires, or how much the superccharged 500 cubic inch produces, but it all goes through a bolted flange less than 7 inches in diameter. From the 2006 NHRA rule book -"For Top Fuel and Funny Car applications, the hole in the motor plate to accommodate the crankshaft flange cannot exceed 7.000 inches."

It appears to use 8 bolts, just like a 1966 Chrysler hemi 426.

http://www.nhra.com/2006/images/news/may/clutch3.jpg

http://static.racingjunk.com/1/ui/7/2/9171986576419.jpg