mechanical coupling with over-torque fuse 1

[rmetzger]

I'm looking for a shaft coupling that will shear at around 150-200 in*lbs of force. I see Lovejoy makes one but it doesn't go down the the torque range I need. Any suggestions?

The application is between a gearmotor and a winch

 

[MintJulep]

http://www.dodgept.com/pdf/catalog/pt_components/2004_pdf/torque_fea.pdf

There is also a spring-detent version, but I can't find it at the moment.

 

[rmetzger]

We went down this road and kept running into corrosion and consistant performance issues. Our application is in an offshore environment which has been creating heavy corrosion (affecting performance) on all the clutch couplings we've tested so far. Instead of constantly battling this we are exploring changing our specs from a clutch to a device that fails at a specified torqe (with replaceable elements to be able to restart the system once the problem is solved).

 

[DesignerMike]

If you can drill the shaft, you could consider a shear-pin. Different diameter pin, diameter where the coupling shears the pin, and different pin materials should give you enough torque range. It all depends on how accurate you need to be, and how often you see an overtorque.
Another option is a magnetic coupling...but their not cheap.
These guys make some neat products. Non-contacting, constant torque, never needs adjustment, generates very little heat.

http://www.rimteccorporation.com/

 

[rmetzger]

I'm looking into the shear pin route as hopefully the scenerio will be infrequent.

 

[MintJulep]

Shear pin is the time-honored solution.

However, if you have corrosiion problems then you potentially have the hub and shaft getting fused together by corrosion products. Thus the pin may never see any load.

Choose your materials for hub and shaft carefully.

Another option is a short section of hollow shaft with a designed failure point, installed between two flange couplings.

 

[maytag]

rmetzger
As you probably know this would be easy if you were to use a hydraulic motor and set the pressure to correspond to your desired torque and then the motor would only stall if and when it reached that set point. maytag

 

[rmetzger]

We tried to push the hydraulic motor route a year ago with no success as our customer is vehemently opposed to them. Power issues have come up (it is a battery operated system) but there is another bias against them that we've been unable to break.

Right now I'm looking at a shear pin / calculated failure joint solution with a custom center element since an off the shelf solution is getting less likely. Because this is serving as a coupler as well as a failure point I'll be making the section between the two hubs out of a damping plastic material so I'm planning on not allowing the metallic elements to contact each other.

 

[diskullman]

I would look into the Lovejoy S flex(or Wood's type coupling) These are known aas "shear" type couplings. Although they have some couplings rated for lower torque, there is no guarantee that the element will shear. Maybe used in conjunction with a shear pin would do the trick. Another thing to consider is a chain coupling type by Dodge or Dalton OSD (Overlaod Safety Device) This combines a chain hub and a clutch hub, which can be tightened to exact slip torque by trial and error. The biggest problem is that maintenance men tend to just tigten them when they slip, rather than looking for the cause of the overload.
There are various electronic overload devices (UST has a shock relay) that will shut down the power to the motor, but does nothing to stop the damaging inertia.

Russell Giuliano
Unique Technologies Associates - Cobra Solid Lubricant

http://www.uniquetechnologies.com

 

[rmetzger]

We actually tested the dalton and found it doesn't repeat well under our conditions due to corrosion (its carbon steel) and a few other factors. I'm a bit hesitant on the clutch systems as those that we have tried haven't held up to our conditions well. Typically our failures occurs when a cable binds in the winch and the system locks - we're trying to prevent the motor from destroying anything expensive as it continues to run.

 

[Tofflemire]

Hi All,

I am not sure if I completely understand your application, but how I am interpreting it, there is a spinning shaft. Does this shaft only spin one direction?

If so, could you make you coupling a 2 piece threaded male and female with either a left or right hand thread with hex flats to torque the two male and female pieces to 150-200 ft/lbs, thus when the forces are greater then the thread unscrews itself.

If the shaft spins both directions then make the couple out of 3 pieces with both left and right threads torque as above.

Tofflemire

 

[rmetzger]

its spins in both directions (cable coming in and out) but your proposal is very interesting and worth looking into. thanks

 

[diskullman]

If you have already tried the Dalton, you may be able to get just the plate sprocket in stainless steel. The last suggestion sounds interesting, but I think corrosion will effect the torque values as well.

Russell Giuliano
Unique Technologies Associates - Cobra Solid Lubricant

http://www.uniquetechnologies.com

 

[rorschach]

Consider using two semi-rigid couplings with a short intermetiate shaft between them that is wasp waisted in the middle to act as the controlled failure point. you'd need to choose an alloy that is corrosion resistant (bronze or SS) so that pits and corosion don't lower the torque capability.

In a previous life we manufactured screw in plugs for a downhole tool that got welded after installation. They were designed similarly to the intermediate shaft idea above so that we had consistent seating torque. When the drive hex twisted off, you were fully seated.

 

[GregLocock]

It's tempting to use a male female socket and a shear pin, but as various people have implied it is difficult to get repeatable failure loads. I think you need an axial separation between the two 'jaws', so I guess I'm thinking like rorschach but maybe using a rectangular strip as the fuse. My experience with wasp waisted shafts is that they'll fail mysteriously.

Cheers

Greg Locock

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

 

[diskullman]

You could also look at Ringfeder ball detent clutches,

http://www.ringfeder.com/RFC Products.htm

or possibly a clutch/brake motor, to disconnect and stop the motor upon overload. You may be better off with this route, as it is not the jam that causes the damage, but the motor inertia.


Russell Giuliano
Unique Technologies Associates - Cobra Solid Lubricant

http://www.uniquetechnologies.com

 

[rorschach]

BTW, if you do go the shear pin route, from experience with shear pins in downhole packers, I've learned that bronze/brass pins are far more repeatable than using other materials. Discussions with some metallurgists I've spoke with credit this to copper's HCP crystalline structure. It is relatively insensitive to cold work level, it will shear out at about the same force no matter if it is half-hard or full hard. brass, being soft is also easy to remove from steel after it shears out too.

 

[sgwhip]

This sounds to me like a really tough application for a shear pin coupling given the stops and starts, running in both directions, and corrosion. Reliability will be horrible. Have you considered an electrical overload device? Personally, I'd have to consult one of my EE buddies, but this would work similarly to matag's suggestion of a hydraulic motor with pressure limits. Further, there will be no shear pin for the operating folks to replace with a grade 8 bolt at midnight.

 

[DesignerMike]

The more I read this thread, it is a perfect application for a magnetic coupling. If you get the right model, there won't be any bearings, no wear parts, and no contact what-so-ever, never need service, and never wear out.

http://www.rimteccorporation.com/

(and there are others tood)

 

[sgwhip]

Cool suggestion.