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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?

7 Comments

  1. kim fawkes on March 12, 2012 at 10:31 pm

    Hi, I was involved in investigating a bolt that was undercut in the machining process which reduced the diameter of the shank, predisposing the bolt to fatigue failure. The ‘special bolt’ formed part of the actuator assembly on an aircraft. When it failed through vibration and cyclic loads it caused seperation of the fork end to the control rod assembly as a result of which the aircraft suffered elevator failure.
    Can you give any comments on this? Cheers Kim Fawkes



  2. Stan Riddle on March 13, 2012 at 3:54 pm

    Thanks for dropping in, Kim! I am not a licensed A&P mechanic, or an aircraft engineer, so I can not, and would not give any comment or recommendation for your application.

    Our recommendations are for general industrial equipment hold down bolts, for shaft alignment purposes, and are based on ASTM specifications for tensile and shear strength of bolt diameters.

    I would recommend contacting an aircraft or metallurgical engineer for your application. Sorry I couldn’t help.



  3. Mark R on March 28, 2013 at 5:54 pm

    Slotting the base is the method I prefer. If the base is threaded them slot the feet. Slotting with a carbide burr tool removes less metal than over drilling the holes



  4. Omer on February 15, 2015 at 10:13 am

    Dear sir,
    We have a problem during installation of new centrifugal pump. The motor came without coupling, it was out of shipment , then when it came and tried to measure the distant between flanges to set up the coupling we found miss matching between the coupling holes and hub bolt’s holes. We started to remove motor towards pump to achieve the distance, bolt bound problem was waiting for us. the coupling type is max dynamic (omega flex). One guy suggest to make new holes in the coupling or drilling the old holes for the coupling, second, we suggest to go for should the motor and drilling out motor feet since it’s not threaded and make a slot. which solution do you prefer to go with? or may you have another idea? please show your opinion. For your information we tried to move pump to the last limit but the issue still same.



  5. Stan Riddle on February 16, 2015 at 8:00 am

    Omer, I would contact the machine manufacturer, as it should be under warranty. If you drill into anything, you may void the warranty.



  6. Omer on February 16, 2015 at 8:59 am

    thank you, you may support me with your opinion view. we talked to the supervisor who has the responsibilities on the pumps and he approved the second solution and he was a way to drill a hole in the coupling. But i still want to know your answer. regards



  7. Stan Riddle on February 16, 2015 at 12:26 pm

    Omer, without seeing the set-up I could not offer a knowledgeable opinion. But I would be hesitant to drill into the Omega coupling. But since I do not know any details, I suspect the supervisor is making a good call. Thanks!