September 20, 2018
Installation Errors Part 4 – Hold Down & Jacking Bolts
By Tom Shelton
Correct fastener torque is important for proper assembly of rotating machinery components to machine bases. Proper torque insures that the fastener will perform as designed. This will in turn help your machine to perform as designed. Hold down bolts and jacking bolts are typically manipulated while performing a precision shaft alignment. These are often the most ignored or least understood aspects of a shaft alignment. To decrease overall vibration in a machine system and increase reliability, precision skills regarding fasteners is paramount.
Hold down bolts are designed to keep the machine in position after a precision alignment is performed. Jacking bolts are designed to help position a machine during the alignment. Jacking bolts are not designed nor should be used to maintain the position of a machine, that is the job of the hold down bolts.
Let’s look at the design intent of each type.
Hold Down Bolts – are to hold a machine in its position. As with all bolts, hold down bolts have a specific torque value to achieve the designed clamping force that is necessary to hold a machine in place. The material and strength grade are also part of the design of the system. The composition, thread pitch and torque value determine the performance specs of the bolt(s). Changing from a SAE grade 5 to a Stainless Steel 304, the properties of the bolt and its capabilities change. They may not perform to the needs of the machine. If you change the bolt composition, you must verify the performance specifications match the designed needs of the machine. For instance, you would never use a brass bolt and expect the same performance as a SAE grade 8 bolt.
The composition of the motor base may also be a critical part of choosing the correct bolt. In a threaded base such as a cast iron pump base, using a bolt of a softer grade (than the base) will help prevent destruction of the internal base threads. The softer bolt threads will “give” if too much torque is applied. This can be acceptable IF the bolts will perform to the necessary specifications required for the machines.
Some chemical pump bases use threaded brass inserts in the composite base material. Over torqueing a hardened bolt will strip the insert which typically CANNOT be repaired.
Jacking Bolts- sometimes called positioning/push bolts – are to be used to move a machine to a desired position. They are not to be used as hold down bolts. There are several reasons for this.
- Thermal growth will occur in all directions. If you create a fix point or limit growth in one direction, it WILL grow another direction.
- Changing the tension on a machine can alter vibration frequencies that sometimes interfere with monitoring equipment set points and sensitivities.
- Shock modulation or base issues may be detected in vibration analysis reading resulting in wasted time and resources determining the source.
Jacking bolts also have a specific design. Typical jacking bolts are composed of hardened steel. They have a square head and an undercut or cupped “business” end. The cupped end is there to accommodate the mushroom effect that can occur from rotating the bolt against the motor foot.
A standard Grade 5 bolt is typically too soft to use as jacking bolt and will mushroom the bolt threads. This typically destroys the female threads of the jacking bolt mounting block as you remove the jacking bolt from the base. I would recommend using a lock nut or removing jacking bolts from the base after a precision shaft alignment is completed. Apply anti-seize to some standard bolts and install them in place of the jacking bolts to protect the threads of the jacking bolt mounting block.