September 6, 2011
Is it OK to Undercut Bolts?
By Stan Riddle
This is probably one of the most common questions we are asked during our alignment training classes. And the best answer might be, “Sometimes, if you must, maybe!” Let me preach on it.
Undercutting bolts is a method used to align machines that are bolt bound. A bolt bound condition occurs when the movable machine cannot be moved horizontally to a degree sufficient to align it to the stationary machine. This most often occurs due to inadequately aligning the machines before other items, such as piping, are installed. Undercutting the bolt reduces its shank diameter, allowing a little more room in the motor foot hole for alignment.
The best method to prevent a bolt bound condition is to rough align the machines before they are installed. Slide both machines as far to one side as they will go and mark the base of the foot. Then slide the machines as far in the other direction as they will go and mark that side. Position both machines in the middle of the two marks and align. This gives you the maximum amount of “hole” in both the driver and driven machines.
On machines already in place, this may not be possible. If you must use undercut bolts, please consult your engineering department guidelines. We offer these guidelines as an opinion only:
- Use the Fixturlaser XA’s Feet Lock™ program to make a slight adjustment to the stationary machine to move it toward the movable machine. If you do not have the Feet Lock™ program, this can be calculated on graph paper. This will allow the maximum strength of the bolt as designed.
- If you must use an undercut bolt, it is a good idea to use manufactured hardened undercut bolts. They are designed for this application.
- If the bolt does not thread into the base and a nut is used underneath, you may be able to drop to the next size smaller bolt, depending on the mass and start-up torque of the machine. Again, consult your engineering department before doing this.
- If you MUST remove metal from the bolt, and this is approved by your engineering department, bear in mind that removing just the thread diameter will not weaken the bolt’s tensile strength. But if you remove additional metal from the bolt shank, you may weaken the bolt considerably. As an example, a ¾” Grade 5 bolt turned to 5/8” will decrease the shear capacity by approximately 30%.
While undercutting a bolt may make the alignment easier, it may cause more problems than it corrects. Make every effort to align your equipment with bolts sized by the designer.
What is your site’s policy on undercutting bolts? How are you solving bolt bound issues?