Shear Bolt Analysis
- This page shows the feasibility of a safety hub design for the Briggs & Stratton engine meant to deal with extreme overload or seizing Scenarios. The hub will lock to the engine drive shaft via keyway and the smaller sprocket will mount to this hub via bolts designed to shear in said overload scenario. The sprocket will receive a delrin type bushing, but no keyway, allowing it to stay located and continue to spin on the shaft once it has broken away from the hub, but not actually driving anything at that point. This hub design can be seen below.
- The team has done preliminary calculations (seen below) pointing towards using two grade 2, 1/4"-20 threaded bolts, necked down to approximately 3/32" diameter. This design would intend to shear the bolts around 43 lb-ft of torque, preventing the load cell from being overloaded. This value is about three times the expected out-put of our Briggs engine, but still within the operating range of typical 525 chain & sprocket applications and well under the dyno's capacity. For further validation, the team designed a test fixture as pictured below. The fixture is designed to mount in an Instron style test machine on campus. Obviously the hope is our test fixture will validate our hand calculations with real world numbers. Depending on the accuracy of initial test results, the team will use trial and error to determine the location and best amount of material removal to result in correct repeatable shear bolt values.
Theoretical Calculations to determine hub shear bolt breaking torque:Givens/Assumptions:
- Max Torque is based around not overloading the Load Cell:
- Lever arm length = 1.33 ft (16")
- Drive Ratio = 3.88 (66T / 17T)
- SM-150 load cell capacity = 150% rated (150 lbs)
- Conservative shear strength theory is approximately 50% of the minimum tensile strength.
- Minimum tensile strength taken from www.boltdepot.com
- 150% * load cell rating = Max load cell capacity
- 1.50 * 150 lbs = 225 lbs
- 225 lbs / (16"/12") = 168.8 lb-ft
- 168.8 lb-ft / 3.88 = 43.5 lb-ft max torque at Briggs
- Fbolt = T / r
- Fbolt = 43.5 lb-ft / (1/12) ft
- Fbolt = 522 lbs force on shear bolt equal to 43.5 lb-ft torque at Briggs
- .50 * (57,000 psi) = 28,500 psi shear strength
- Need to aim above 28,500 psi of shear stress to break bolts.
Case 1, 7/64" Dia:
- shear stress = Fbolt / Area bolt
- shear stress = 522 lbs / (2 bolts*(pi*(7/64")2/4))
- shear stress = 27,779 psi , result: Will not break.
Case 2, 3/32" Dia:
- shear stress = Fbolt / Area bolt
- shear stress = 522 lbs / (2 bolts*(pi*(3/32")2/4))
- shear stress= 37,810 psi , result: Will break!
Case 2, in reverse:
- (2 bolts*(pi*.(3/32")2/4)) * 28,500 psi = 393.6 lbs force
- 393.6 lbs * (1/12) ft = 32.8 lb-ft
32.8 lb-ft of torque at Briggs is theoretically where the 3/32" shear bolts should break
Experimental Results to determine hub shear bolt breaking torque:
- Started by making qty 3 each of 1/4"-20 grade 2 bolts reduced to the following diameters:
- 3/32", 7/64", 1/8", 9/64", 5/32"
With the help of Tim Landshoot, the P10221 team was able to perform the shear bolt testing ahead of schedule. The MTS Tensile Compression equipment in the Mechanics Laboratory was used for the following Tests. The tests were done using the 2500lb load cell configuration in the machine, with 10HZ sampling, and a .05 in-min separation rate. These were all chosen values to fit our application best.
Test Fixture Details:
- Area of clamped faces equals approximately half of sprocket/hub clamped faces.
- Widths of test fixture pieces representative of sprocket and hub, .295" & .25" respectively.
- Hub side of fixture threaded and sprocket side of fixture is a through hole, representative of actual layout.
- For the results shown, bolt torque specs were kept the same (5lb-ft)
- Below you can see the completed Test Fixture mounted with a shear bolt in place:
Summation of results:
- Ran qty 3 each of the following sizes:
- 3/32", 1/8", 5/32", 1/4", and 1/4" grade 5
- Tabulated results of the different diameter's three test average:
- The Excel graph of load vs. displacement of the different diameter's three test average:
- With good repeatability and trends as expected, we then honed in on the desired reduced diameter of 3/32". The data from the three test, as well as the load vs. displacement graph of the three runs can be seen below:
- The repeatability of the three runs was relatively good with variability of under 15%.
Comparison of Analytical and Experimental Results:
- Below is reconfirmation of hand calculations with the experimental break data:
- 381 lbs * (1/12) ft = 31.7 lb-ft
- 31.7 lb-ft of torque at Briggs is where the 3/32" shear bolts broke in experimentation
This represents only 3% error for torque.
- shear stress at break = 380 lbf / (2 bolts*(pi *.(3/32")2/4))
- shear stress at break = 31,436 psi
This represents only 10% error for shear stress.