Set screws keep coming loose on stepper motor coupler.

cmartinez

Joined Jan 17, 2007
8,220
We use lots of sets screws and must adjust them regularly as part of calibration procedures.

The important ones that we don't want working loose between PM's are secured by a second set screw, torqued down against the top of the first one.

IT WORKS GREAT. If you have a deep enough threaded hole
Say, that's a good idea, but I'm sure you meant to say "If the machine has a deep enough threaded hole"... seriously now, how do you prevent the screw on the top from damaging the socket of the screw below it? By not over-tightening it?
 

#12

Joined Nov 30, 2010
18,224
The only thing I ever learned about set screws is: Never let any oil get near them.
Those fan blades can really fly when they walk off the shaft!:oops:
 

atferrari

Joined Jan 6, 2004
4,764
Be warned that I know very little about anything mechanical.

My question: isn't trying to keep those screws in place instead of finding out why they do that recurrently, curing the symptoms and not the disease?

A similar fitting doing what I understand is a similar task, is not suffering the problem. ???

But my ex and her mother always said that I was always wrong. Learnt to believe them.
 

cmartinez

Joined Jan 17, 2007
8,220
Be warned that I know very little about anything mechanical.

My question: isn't trying to keep those screws in place instead of finding out why they do that recurrently, curing the symptoms and not the disease?

A similar fitting doing what I understand is a similar task, is not suffering the problem. ???

But my ex and her mother always said that I was always wrong. Learnt to believe them.
Hola Agustín

The reason why those set screws will come loose is vibration. With time, vibration causes small "shocks" inside the thread that breaks the high friction between the screw and the hole. Regarding screws, you have to think of them as if they were extremely stiff springs. When they're tightened is like you're loading a spring that keeps both parts together. The thread acts as an inclined plane, and the spring remains loaded because of the friction in that plane. When vibration is applied, it can deform that spring (the screw) in a wave-like manner that will travel back and forth along the screw, very briefly causing the threads to separate in small sections of the screw. When that happens, the friction between threads is lowered so much that the spring-like force stored in the screw will start to turn it loose.
The normal remedy for that is to use a special adhesive that will prevent the threads from separating during vibration events, thus impeding the thread from slipping, and the screw from turning.
By placing a small stainless steel valve between two set screws, you're in a way placing another spring between two springs, but also, stainless steel is less elastic than normal steel and will absorb vibration better. That's why I believe that what Kermit2 says must be true.
 

GetDeviceInfo

Joined Jun 7, 2009
2,192
Set screws are not designed to transmit the load. That work is done by a key or bore to shaft fit. The compression fits provided by the devices Max referred to would be your easiest route. Depending on your shaft size, you could possibly drill a spring pin through the assembly.
 

cmartinez

Joined Jan 17, 2007
8,220
Set screws are not designed to transmit the load. That work is done by a key or bore to shaft fit. The compression fits provided by the devices Max referred to would be your easiest route. Depending on your shaft size, you could possibly drill a spring pin through the assembly.
The thing here is, that set screws work best on the keys of keyed shafts than pressing directly on shafts with flats machined on them. If it is the former, the use of an adhesive such as loctite is highly recommended, if it is the latter then it is essential. Using a pin can also work, but it has to have zero play, and should be press-fit or somehow pre-loaded, like in a roll-pin
 
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Wendy

Joined Mar 24, 2008
23,415
The zero play is the hard part. Many couplings have 1/2 half a shaft that interlock (or some other geometry) , and the setscrew is just to help hold things in place.
 

shortbus

Joined Sep 30, 2009
10,045
The reason why those set screws will come loose is vibration.
The couplings used with most of the small machines are the "helical" type. They have a bigger problem with vibration, they 'windup' when torque is applied to them, and 'unwind' when the torque is stopped. They are also only torque rated for turning in one direction, when used in a reversing situation, like a CNC, the rating is only 1/4 of the single direction.
 

cmartinez

Joined Jan 17, 2007
8,220
The couplings used with most of the small machines are the "helical" type. They have a bigger problem with vibration, they 'windup' when torque is applied to them, and 'unwind' when the torque is stopped. They are also only torque rated for turning in one direction, when used in a reversing situation, like a CNC, the rating is only 1/4 of the single direction.
That's why I like to use the collar shaft clamp type in my designs whenever I can

Shaft_Clamp_Collars_1piece_ss.jpe
 

MaxHeadRoom

Joined Jul 18, 2013
28,619
That's why I like to use the collar shaft clamp type in my designs whenever I can

Thats the style in #38.
Incidentally when using bellows type couplers, go for the stainless S. version, it is worth it over the aluminum type, which eventually suffer from fatigue on servo's etc, they are OK on encoders.
Max.
 

Robin Mitchell

Joined Oct 25, 2009
819
@spinnaker

I have those screws too and I too had this problem. Until I got a hex key and turned that thing so tight it has never come loose since. The 3020 needs tough love. My machine has had hammers to it and is still living strong! To be honest the hammer was to get the spindle out and it was a small hammer :D
 
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