Spring-Holding Tool Stud Designs


I believe the Mac Tools style spring-mount stud shown in my holding tool plan is the best design. However, as it might not be possible for some people to have such a stud machined, here are some suggestions for an alternative.

You could take a 7/16” Grade 5 bolt and grind the head round to  9/16” diameter. There are two drawbacks to this method:
  1. You would have to quench the bolt continually to keep it cool during the grinding process, because overheating the bolt will affect its temper and reduce its strength. This is a critical safety consideration!
  2. Once the head is round, it will be difficult to tighten the bolt into a threaded plate (see below), and it would be impractical to grind the head after installation.
Another possibility is to use a Grade 5 flathead bolt (aka “cap screw”) between 5/16” and 7/16” shaft diameter. These bolts generally have a round, tapered (“flush”) head with a recessed socket, or drive well, for a square drive or Allen wrench.

In selecting bolt size, there are trade-offs:

 
<—— A thicker bolt is stronger, but it will take more grinding (the caution against overheating applies) to reduce the head to 9/16” diameter. It might also be necessary to square off the taper from the head to the shaft to create a good lip, taking care not to grind too close to the bottom of the drive well in the head—if the well is very wide and/or deep, the area between the bottom of the well and the notch could be a weak link.   
A thinner bolt will not be as strong,
but it might require little or no grinding. ——>
 



Forces at Work

In this application (holding a partially compressed Rambler front coil spring), almost all the force exerted on the stud is in the shear plane. Some tensile force is also possible, so it is important to secure the stud to keep it from being pulled out of the plate. Since no pressure is likely to be exerted that would press the stud back toward the plate, securing the stud in the compression plane is merely a matter of convenience in handling and placing the tool. Figure A below illustrates the forces this stud will encounter.




Mounting Methods

Figure A above shows the Mac Tools style flanged stud in a 1/4” steel plate. The stud shaft is 7/16” thick, and the threads begin where the stud has passed through the plate. The inner flange secures the end of the stud opposite the nut (not shown).

Figure B shows another approach. A 5/16” Grade 5 flathead bolt passes through a 1/2” steel plate. The bolt has a 5/8” long shoulder, so the threads begin where the bolt has passed through the plate. Here, a nut will have to be secured to the back of the plate with epoxy or a spot weld. (CAUTION: If spot welding, take care not to overheat the bolt! This will affect the temper and reduce its strength.)

Figure C is a workable alternative to Figure B. The bolt’s shoulder is shorter; the threads begin about halfway into the plate. No nut is needed.

Figure D shows a bolt with minimal or no shoulder. The thread cut creates a potential fracture point right where the shear force is greatest (red arrow).
This design has been tested to 3,500 pounds (875 pounds at each of four bolts) without failure, so it should be OK for this application. Even so, IMHO it is not optimal.
   



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