Estimatin g Impact Force from Rotating Shaft on Collar

[jayelec]

Hello All,
Some background information: I'm a co-op engineering student and recently my supervisor asked me to do a bit of research on a solution to a problem we had. Essentially, a cardan shaft connecting a motor and a gear box failed at a joint during operation and caused some extensive damage (luckily no one was hurt). A solution to this was to design a collar placed midway of the shaft to "catch" the shaft if it breaks and prevent damage to the surrounding. This shaft is rotating at high speeds so we are assuming precession plays an important factor as the shaft would hit the side of the collar instead of falling straight down if it breaks at a joint.

An important aspect to know before designing the collar is an order-of-magnitude estimation of the impact force from the shaft to the collar. One approach that I have been thinking of is using an energy method where we find the rotational kinetic energy due to precession from the shaft and equating that to the strain energy of the collar when it deforms from impact. Keep in mind that there assumptions that will need to be made (the collar will elastically deform, the shaft is rigid, the collar is circular, there is clearance between the shaft and the collar, etc.). My question is: is this the right approach to find an estimation of the impact force or is there another approach (maybe better) that I can take? If anyone can share any insight on this that would be awesome! I've also included a quick diagram to give a better idea of the shaft and collar.

 

[GregLocock]

Something like 10 times the whirling force, defined as omega ^2*(mass of shaft+coupling)*r*leverage, where r is the lateral deflection of the cg of the assembly allowed by your clearance, and leverage is some complex relationship between the cg location and the restraint location.

Make your restraining strap from a ductile steel strip (ie boring old mild steel), put some form into it so it plastically deforms when hit, and fit a limit switch to kill the motor when the shaft breaks. With this choice of material there is no point in finessing the design, make it bigger than it needs to be, unless you are stuck for a mass limit or something.

The key to restraints is to make them robust, strong, yet plastically deformable.

You might consider fitting more than one of these, or even enclosing the driveshaft in a cage, depending on what it is made of etc.

Cheers

Greg Locock


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[desertfox]

Hi jayelec

Whilst we do a similar thing at the company I work for to what you are trying to do, I would be more inclined first to find out more about why the shaft failed in terms of location of failure, type of failure and how long its been in operation.
On the sketch you state you have knowledge of the shaft details but you've shared none, so if you can post the details it might help us to help you.

 

[Tmoose]

concerns of u-joint breakage are behind page 10 here -

http://www.nhra.com/UserFiles/file/Unleashed/2011 NHRA Rulebook General Regulations Section.pdf

The legend is if the front joint breaks the car can will "pole vault" when the tube drops and digs in to the road.

those guy's driveshafts have pretty good reason to break once in a while.

As do these.

https://www.youtube.com/watch?v=HmlQokVYRoE

https://www.youtube.com/watch?v=hQvn4K7DnEk

 

[tygerdawg]

The Fox is right, you know.

My first thought of your limited information is that this seems a bad alignment situation or poor coupling installations might be your root cause, but you're chasing a patch solution. Careful there, buddy. Long shafts rotating at high speed can be set up properly, but there isn't much room for error. Especially if your Crafts people are a little negligent in their Millwright skills. Constraining the shaft as you wish to do may help (the old problem of a power lines' natural frequency changed by more birds sitting on the line). But what other problems does it cause (maintenance, etc.)?

Would have been interesting to see pictures of the actual failed shaft. That could be very telling.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[IRstuff]

The good news is that the shaft will still be rotating when it impacts the collar, so it'l probably skitter around the opening for a while, lessening the magnitude of the impact force.

TTFN
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[tbuelna]

Using a collar mid-span about the shaft is not a good way to constrain the shaft in the event of an end joint failure. The shaft loose at one end would simply whip around the inside of the collar, and even possibly break free of the remaining end connection. It would be better to provide a full length enclosure to constrain a shaft flail event. A steel housing that has a close fit around the rotating components would be a good approach.

I have personally witnessed just how much damage a flailing high-speed (15K rpm) steel shaft can create in a test cell. So I think it would be a good idea for you to install a sturdy full containment structure around the driveshaft in your test cell. Weight should not be an issue, so make it from thick and sturdy welded steel.

 

[GregLocock]

When we whirl tested driveshafts on the track we just use 3 off 2 inch wide straps to close off the bottom of the transmission tunnel. That'd have been a ~48" shaft at around 6000 or 7000 rpm.

Cheers

Greg Locock


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