Double Shear Pin Failure

[Spaulding9717]

I have a shear pin on a snowblower auger that is bending before it shears. I'm currently trying to figure out the max shear force the pin can take, having the materials given for the pin. The thing is, I'm not sure if I need to account for the bending of the pin or just try to solve for shear force. Any ideas?

 

[Tmoose]

Is it a genuine replacement shear pin with at least one deep notch in the pin body inline with the auger/shaft interface?

https://e2x3s6i4.ssl.hwcdn.net/main.../MAGNE003/ProductLarge/snowblowershearpin.jpg

 

[desertfox]

Hi Spaulding9717

If it were me I would just check it for shear.

 

[Jboggs]

If this is a standard shear pin installation, then the pin and related components should be designed and installed so that bending of the pin is nearly impossible without deforming related components. What else is deforming besides the pin? The theory is that the two components the pin locates are very stiff and very close. If they are very stiff and very close the pin will shear before they will deform. The closer that gap gets to zero the less bending load the pin will see. It will only see a pure shear load, like the jaws of a pair of scissors. The two components should almost be touching. I would guess that there is a large gap between the two pinned components, thus causing an excessive bending load on the pin.

 

[tbuelna]

I agree with Jboggs. A properly designed, manufactured and assembled double shear pin shaft connection should not produce any bending in the pin. The pin should have a tight fit in the shaft holes. There should be minimal clearance between the mating shaft ends at the shear interface. And the pin should be supported along its entire length by the mating shaft ends.

If the shear pin is bending, that means there is excessive clearance in the joint at assembly, or some part of the joint is yielding under operating loads. Maybe check the pin and shaft holes for edge bearing stress.

 

[ESPcomposites]

Well, there has to be some bending in the pin. The only way to avoid bending would be the have the entire shear distributed over a very small bearing surface area (infinitely small), right at the shear planes.

In reality, the shear load is distributed over some effective bearing thickness. Once you do a free body diagram for that, the eccentricity allows you to calculate the amount of bending. There are various methods that are used, but the problem is determining the effective bearing thickness (and hence moment arm). First order calculations use half the lug thickness, but those are usually too conservative. If you can show acceptance for that, then you are done. The next order solutions use the bearing strength to determine the effective moment arm. If you have a copy of Bruhn's book, Chapter D.1 has some calculations. You can also look at ESDU 91008:

https://www.esdu.com/cgi-bin/ps.pl?sess=unlicensed_1150411085108bwc&t=doc&p=esdu_91008a

But the you don't "interact" the bending stress of the pin with the shear stress. This is because the peak bending stresses occur at a different location along the pin than the max shear stress. So in that sense, what Jboggs and tbuelna can be true. Pin bending is not usually critical for well designed connections. If there is no gap between the shear planes, it should be shear critical. In a situation where there is a gap, it can be bending critical. This can occur when a design uses a spherical bearing and allows for angular travel. A gap is required to allow the joint to rotate to some degree.

Brian

http://www.espcomposites.com

 

[Tmoose]

the OEM shear pins/bolts in my consumer grade snowblower break from time to time. Stones from the town plowing the dirt road end up at the mouth of out driveway, etc.
One end has a large diameter that engages an enlarged diameter hole in the tubular auger shaft. Just below the enlarged diameter the the bolt body diameter is reduces significantly. The other end is nominal bolt size, as are the holes in the solid drive shaft, and the opposing hole in the tubular auger.

http://i5.walmartimages.com/dfw/dce07b8c-f7e7/k2-_0e5bc2e8-62ef-4dac-b2cb-13d26d7e3e9c.v3.jpg

By now the large and small holes in the tubular auger shaft are deformed some. After a shear pin/bolt failure, The stub captured in the solid shaft usually shows significant bending at the un-notched end. I believe that was the case on the first shear pin to fail.

 

[Spaulding9717]

Thank you for the responses, the problem has been discovered as a combination of the gap between the auger and the axle, and also the diameter of the hole for the pin being to large which leaves a lot of room for the pin to move around and bend.