Vibrating Motor Required Bolt torque

[msii]

Hi all,

We have an unbalenxed vibrating exciter for a vibrating screen and require to know what bolt torque is required to fasten the vibrating motor to the screen frame?
Considering the point that the motor is an old one and the manufacturer is not available is there any way calculate the required bolt torque based on material of the motor casing and screen and diameter of the bolt?
Known:
Material of screen frame and thickness
Material of motor casing and thickness
Diameter of the fastening bolts and quantities
Excitation force of the motor
Thanks

 

[TugboatEng]

There is nothing special about your application. The standard bolt torque tables should be applicable. If loosening is a problem consider using wedge type lock washers. If breakage is a problem look in to increasing grip length.

 

[drawoh]

msii,

Your requirement is that your bolts do not come loose. Specify the highest torques you can. Use high strength bolts so that you can specify more torque.

--
JHG

 

[msii]

Thanks guys for replying.
I m looking to find the minimum torque that i can use to avoid breakage of the casing and in a same time can keep the drive in place.
I m thinking of using bolt grade 8.8 for this motor but using the torque specified in the standard bolt table is too high and may break the casing.
Any thoughts?

 

[desertfox]

Hi msii

So what grade are the existing bolts? I would be tempted to use 70% ofthe maximum torque of the existing bolt grade. If you have enough information on the materials strength properties you could calculate the torque for the joint see this link

https://roymech.org/Useful_Tables/Screws/Preloading.html

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[TugboatEng]

I suspect your casing is breaking due to the bolts coming loose first. Do you see signs of fretting corrosion? This would be indicative of bolt failure.

 

[CWB1]

Are you actually failing parts or simply concerned over the possibility? If failing parts are you ripping out threads, failing the clamped part (motor mount?), or seeing another failure mode? Details are important.

In any case, you need to take 10 mins and run a standard bolted joint analysis. Torque charts are a bad crutch for non-critical joints, not something any engineer should be referencing with any regularity. If these are older parts then I would also do a thorough inspection of the mating surfaces and threads for wear and damage.

 

[dvd]

Good info on fastener joint analysis

Think about safety wiring the fasteners

 

[TugboatEng]

Tie wire keeps the fastener from falling out once it's loose. It's a poor choice to prevent loosening.

 

[msii]

Hi All,

For a similar applications and a different motor fabricator, the bolt torque that the supplier provided us was 50% of the yield stress of the bolt. So, I'm a bit concern about of probability of the casing breakage if I want to torque it up to 70% yield.

thanks,

 

[3DDave]

The amount of load the bolt can accept has zero to do with breakage of the other part. You need to figure out what the motor mount can take - first. Then figure out the force the vibration will apply to the mount. If the mount on the motor breaks in this use the bolt doesn't matter.

 

[desertfox]

Hi msii

I would follow the suppliers recommendation of 50% however be aware that torquing up bolts is by no means accurate you can be +/- 25% out from the set torque figure given, depending on whether or not the bolt is lubricated and the surface finish of the mating parts etc etc

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[dhengr]

Msii:
Are the nuts and bolts coming loose, with the vibration? Once you’ve clamped the motor down, additional bolt tension is sorta meaningless, as long as you are not breaking bolts. And, as mentioned above, torque is not a very good method of determining bolt tension, except under very controlled conditions. “50% of the yield stress of the bolt,” is not a torque, rather, it leads to a bolt tension, given a bolt grade and size. Are the base of the motor and its associated mounting castings flat, planar and true, within reason? What is their strength in terms of potential breakage? Is the mounting base on the structure flat and true? The planar and flatness issue relates to having to really crank one motor foot down because it is out of plane with the others. Shims might correct this. Is the structure strong enough and stiff enough so that it isn’t fluttering all over the place due to the vibration, and unduly flexing the motor base at a weak point? Are you actually breaking motor casting, or just worried that you might?

 

[dvd]

TugboatEng - disagree

 

[3DDave]

In the event the material clamped by the bolt deforms it's possible for the entire preload is lost - lockwire is to keep the now-loose fastener from escaping and either getting somewhere it should not be or to keep the fastener in the hole where some amount of shear load can be carried. Lockwire cannot produce additional spring load to compensate for the loss of preload due to embedment, differential thermal expansion, or wear. Unlike locking compound it will retain the head of the fastener if the fastener fails due to fatigue or overload - possibly covering up a dire situation from casual view.

The primary function that lockwire serves is to remind the installer to check the fastener is actually tight - it's tough to install when the bolt or nut is loose without noticing that fact.

 

[Tmoose]

Have any motor casings actually broken?

If they have, I would be looking at the mounting faying surfaces not being flat and co-planar.
I do not recall ever seeing a motor whose mounting feet or flanges are cantilevered to leave a gap between contact points and the bolted flange.

Then, as other said, bolt loosening do to improper installation and subsequent embedment etc and eventual fastener loosening is most likely.
Damaged faying surfaces, spring lockwashers, and inadequate grip length are just looking for trouble.

 

[TugboatEng]

That is a good point a out the faying surface. I had bolt breakage on a flange that has the bolts tightened against an as cast surface. The surface had draft and the bolts would fail due to vibration. Spherical seat washers solved the problem. The manufacturer has since began spot facing the bolt holes.