Adjustable chocks have been around for years and are a useful way to accomplish parts of machine mounting and alignment. In some circles, they can have either a great or bad reputation. A lot of that reputation may depend on the application and how the chocks were installed.
Adjustable Chocks vs. Shims
First, let’s discuss why a company might want to use an adjustable chock for machine mounting, instead of shims:
- Adjustable for height. This means not having to stock lots of shims, in different sizes and thicknesses, to accomplish vertical adjustments in alignment.
- Beveled top washer. This accommodates issues with feet not being parallel (up to 4 degrees) and eliminates the need for step shimming angled corners.
- Easy Soft Foot corrections. When an air gap is found, simply fill the gap by adjusting the chock to fill the gap.
- Reduced inventory. Instead of several shims in a kit for each piece of equipment, just reuse the existing chock for adjustments.
Now, let’s discuss why a company might not want to use an adjustable chock for machine mounting, instead of shims:
- Lack of contact between mounting foot and base. How can this round element take the place of a full-footprint shim for secure mounting?
- Transmission of inertia. Without the solid contact of that full-footprint shim, the energy will never be transmitted to the Inertia Block in the base; therefore, the equipment will shake itself to pieces.
- Locked up chock. Once they have been in service for a while, they lock in place and have to be replaced for future alignments.
- Failed chock. Upon inspection, the chocks have been found to be collapsed under the foot, and there is a gap/Soft Foot condition.
The Cons of Adjustable Chocks
By looking at each of those concerns, answers can be found for how to mitigate the concern and better understand how adjustable chocks can (and cannot) be used. The design and engineering of these devices make them suitable for most applications, but not if selected and used incorrectly.
For the issue of lack of contact area under a foot to the mounting base, looking at the product catalogs show any number of configurations to increase the surface area. Using more than one at each foot, or having two under a foot but staggered. The simplest rule is to use the largest chocks that can fit, but with a catch. The top surface must have at least 75% of the surface area covered with the mounting foot, and the bottom surface must have 100% of the surface area in contact with the base. Both of these surfaces must be clean.
This leads to the next issue, the transmission of inertia. If the correct size chock is selected, and the above rule for coverage is observed (75% top, 100% bottom), then the energy should transmit through the body of the chock just fine. The other thing to watch for is cleanliness. Both the bottom of the mounting foot and the top of the base should be a machine finish, with a minimal primed surface for corrosion purposes. Any amount of paint, dirt, or debris can make for an uneven surface that could allow the bottom of the chock to not sit squarely. Sometimes, a bit of light sanding can go a long way to promoting proper machine mounting.
Now, for the locked-up chock. Oftentimes, comments are made that the machine being aligned is unable to be lifted by the adjustable chock. That is because the chock is designed to lock under load. It is not designed for lifting or lowering the equipment. Normally, equipment that is designed correctly will have vertical jack bolts, and this is what is used to establish the correct elevation for the machine. Beyond that, finding the chocks unable to rotate after being under a piece of equipment normally can be attributed to dirt and debris in the threads. It is a very common practice to lift the equipment on the vertical jack bolts just enough to remove the chocks. Thorough cleaning in a general solvent can remove the particles that restricted movement. After the cleaning, a thin coat of appropriate lubricant (often a specific compound recommended by the chock manufacturer) will help ensure movement. Protecting the cleanliness of the chock after the alignment has been accomplished can be done with a heavy protective spray. Anything that seals moisture and debris out is good, as long as it does not trap moisture.
Lastly is the concern of collapsed chocks, which has been a topic of much discussion lately. The easiest way to explain this problem goes back to proper training. The technician performing the alignment needs to be mindful of how an adjustable chock is designed to work. The function of that chock is to support the machine. Prior to torquing the hold-down bolts, the machine needs to be resting on the chocks, not on the jack bolts. The procedure boils down to using the jack bolts to establish the correct elevation, spinning the adjustable chocks up to firm contact with the bottom of the machine, backing off the jack bolts FULLY, then torquing the anchor bolt properly. If the jack bolts were left with tension, holding the equipment up, the adjustable chock never saw enough force to properly lock the threads. If the jack bolts were released prior to the torquing of the hold-down bolts, and one of the adjustable chocks has independently collapsed, this indicates that the load never was present on that chock.
The purpose of adjustable chocks is to facilitate proper mounting of equipment, more efficient alignment operations, and an economical replacement for traditional shims. While they might not work for all applications, with proper implementation chocks can work for 90-95% of applications where they can make life easier for the technicians working directly with the equipment. This requires an open flow of information from the design and installation of the equipment, all the way through to the day-to-day maintenance