I keep breaking bolts

[mwb]

Here is another torque chart with different grades.

Another note; I believe that most equipment uses grade 2 for shear pins. I know of many who figure that upgrading to a higher grade will 'solve the problem'. If you are breaking shear pins it is usually caused by one of two problems; 1. you hit something, 2. the shear bolt was not torqued properly and failed due to fatigue.

Good thread. I like 'screwin' around.

 

[SHREKatimport]

make sure to only use new bolts used grade 8 are stressed. you would not think you would have to tell only new but I see customers everyday with a bucket of used bolts using them.

 

[Eagleview]

How about a photo of this contraption ? It would be good to be able to see the bolt spacings , shank lengths etc to eliminate guess work as to what is happening . If the bolt spacings versus shank length is too small , the mechanical purchase will overcome ANY bolt .

 

[Mace Canute]

I found a pic of one. From a website..."The 15 1/2-in.-long B-33 features a 1 1/2-in.-wide horizontal foot on the bottom." (just to give an indication of size.)

 

[MF RED in MT]

Hey everyone, My MF 2360 snowblower manual says to use a grade 8 gold plated single groove shear bolt for the fan, and to use a grade 5 silver plated double groove shear bolt for the augers. Both grades of shear bolts are a 5/16"-18 x 2.5" long and MF specifies the torquing values for all at 9 lbs-ft. Is there a chart for shear bolt torque values as the charts listed show standard bolt torque values, not shear bolt torque values ? I think I understand if you over torque a shear bolt, you are changing the shearing values? KC

 

[captin_insano]

If the bolt spacings versus shank length is too small , the mechanical purchase will overcome ANY bolt

Yep, pictures needed. I didnt read the whole thread but it would also help to know if were talking single or double shear.

 

[Brad_Blazer]

KC,
For your snowblower application I think they just want the bolts snug enough to retain the nut and rely totally on the shear strength of the bolt to provide repeatable shear performance to protect your machine. Torquing an assembly in shear to a high value will increase the allowable shear load due to the friction between the plates considering the clamp load of the fasteners. If you were to torque a shaft assembly to the allowable value based on the fastener, you might deform the outer sleeve into an oval and make it hard to assemble. There is probably too much variation based on application to provide a torque chart for shear bolts on rotating shafts so I'll say "sensibly snug" is the rule of thumb.

Back to the original issue:
Looking at Mace's pictures, per Iron Horse, maximizing the space between the bolts by going to the bottom of the slot should make a big difference. It may also be possible to add a third fastener in the slot.

One fastener grade not mentioned is the socket head cap screw which has a 180,000psi proof load.

Of course you know that as you make the designed failure point stronger the risk goes up of it breaking somewhere else. It could be anything from attack angle to rocks to lack of a coulter disk causing the failures assuming you are employing the shanks as designed.

My favorite torque chart with Metric sizes and even clamp loads:
http://www.imperialsupplies.com/pdf/A_FastenerTorqueCharts.pdf

 

[MF RED in MT]

Brad, thanks for the shear bolt torque info and also the torque charts, I will insert them into my MF Maintenance binder I have put together for all of my equipment :thumbsup:. KC