Design of a shear pin. What fit between pin and hole.

[gearman1234]

Could any body help me in deciding what fit is supposed to be used between a shaer pin and mating hole. This connection is supposed to drive a torque. I do not know the value, as it is customer design.

Pin is made of material 96 ksi and the driving member is made of UTI 116 ksi. Nominal dia of pin is 8 mm.

Is there any refernce book available on this subject?

 

[desertfox]

Hi gearman1234

If this pin is to transmit a torque then I would use an interference fit.
The "Machinery's Handbook" has information about limits and fits.

regards desertfox

 

[unclesyd]

We used small hardened 1095 CS shear pins on over 1000 pieces of equipment. The pins were 0.061" and 0.078" . The pins were water quenched then immediately drawn to 900°F. The final tempering temperature was determined by testing in double shear to give the desired shear strength. We were trying to achieve a pure shear failure, no tensile.

There were several parameters that greatly affected the point at which shearing occurred.

As per you post the clearance of the pin is very important. We used what was called an “machine fit or slip fit” between the pin and hole. We achieved this by using a long hardened drill bushing, essentially no wiggle.

If using a hardened pin the shearing edge of the hole needs to be perpendicular to the hole with no radius, sharp. A rounded or worn hole will allow a pin to be pulled out enough to lock the two driving hubs by wedging.

Again if using a hardened pin the clearance between the pinned hubs should be as low as possible. Our driving hubs were held in place by a bearing that kept the plates holding the drill bushings parallel and perpendicular to the centerline of the shaft.

The use of soft materials Al or Brass is essentially the same unless using a reduced cross section pin. You still need to keep the pin from bending and wedging the hubs togather.
A reduced cross section pin is usually locked in its holder and installed as a unit. The clearances are not near as critical as you need with a hardened pin designed to fail in shear. This type pin usually carries enough clearance to keep from locking the hubs.

Driv-Lok and Spirol did have some good information on using spiral and solid pins as shear pins.

Another point that is very important concerning the life of shear pin is that any motion other than rotary is very detrimental to the life of said pin.

 

[gearman1234]

Thanks for the information given by desertfox and unclesyd.

Adding more to my earlier post, the customer wants to use a threaded screw as a shear pin instead. So the question arises as to what would be the dia of the unthreaded area and the mating hole so that the bolt will fail only in shear and not in tension. This throws up a question whether the screws should be made specially or should we use an existing threaded screw and use it as it is.
I checked the body dia tolearnces and thread dia tolerances in Machinery's Handbook and they are quite open and would not give the desired fit betweeen the body dia and the hole.

 

[desertfox]

Hi gearman

You might be better using a shoulder screw these have an accurate plain shank and a thread at the end of a smaller diameter than the plain shank,this will allow a close fit for the shear portion and allow the parts to be fastened together.

regards desertfox

 

[strokersix]

Agricultural machinery often have cotter pins used as shear elements for overload protection in drive systems. Commodity cotters can have widely variable material properties. This is not much of a problem when the cotters are used for retainers but it can be a big problem when attempting to use them for drive system protection. When I say widely variable I mean observed shear torque range of 4:1. Advise to consider material specifications of whatever shear element chosen in addition to joint geometry.

 

[gearman1234]

Thanks for the info.
I referred to catalogues of different fastenere manufacturers. They have grade 12.9 cap screws and we need hex head bolts/screws. So I am planning to specify a design similar to shoulder screw but with hex head and with property class 8.8. This is so since we have already chosen the material for the drive yoke (which takes the bolts) to be having UTS at 120 ksi and grade 12.9 has UTS more than that value and so the bolt will not fail in the event of excessive shear stresses. But if we use bolts with property class 8.8 then that will be the weakest link.

Dia of body will be more controlled to 8f6 and the corresponding hole size would be 8H8.