Adjustable chocks have been around for years and are a useful way to accomplish parts of machine mounting and alignment. In some circles, they 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.
- Spherical top part. This accommodates issues with feet not being parallel (up to 4 degrees for some manufacturers) with the foundation or skid which eliminates the need for step shimming.
- Easy Soft Foot corrections. When an air gap is found, simply fill the gap by adjusting the chock to fill the gap. (Zero Soft foot)
- 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 surface between mounting foot and base. How can this round element take the place of a full-footprint shim for secure mounting?
- Transmission of Energy. 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.
- Loose chock. Upon inspection, the chocks have been found to be rotated down 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 surface 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 to use is to use the largest chocks that can fit, but with a catch. The top surface must cover at least 75% of the surface area of the mounting foot, and the bottom surface must have 100% of its surface area in contact with the base. Both of these surfaces must be clean.
This leads to the next issue, the transmission of Energy. If the correct size of the chock is selected, and the above rule for coverage is observed, then the Energy will 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 clean – reasonable steel-of-the-truck finish (corroded, excessive mill scale, or moon crater need attention, with a minimal primed surface for corrosion purposes. Any amount of paint, dirt, or debris can make for an uneven surface that could result in the bottom of the chock not sitting squarely. Sometimes, a bit of light sanding can go a long way towards promoting proper machine mounting. Check with chock manufacturer for surface finish recommendations.
Now, for the locked-up chock. Oftentimes, comments are made that the machine being aligned is unable to be lifted by the adjustable chock. This is a misconception. The threaded 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, or, in their absence, use a hydraulic pancake jack or other suitable lifting devices. Beyond that, finding the chocks unable to rotate after being under a piece of equipment can usually 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 restrict 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 alignment can be accomplished with a heavy protective spray. Anything that seals moisture and debris out is good, as long as it does not trap moisture (just to note: whatever you put on it will need to come off at some point to allow the chock to be reusable – be judicious or better yet contact your chock supplier).
Lastly is the concern of loosened 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 all of the adjustable chocks up to firmly contact with the bottom of the machine, and then perform the final tightening. (Fit all chocks at the same time!!) Back off all lifting and lateral adjustment jack bolts FULLY, then tighten the anchor bolts to the proper torque in the sequence specified by the equipment OEM
Do not tighten the anchor bolts with the jack screws or lifting bolts under load!
The purpose of adjustable chocks is to facilitate proper mounting of equipment, more efficient alignment operations, and a viable replacement for traditional shims. While they might not work for all applications, with proper implementation chocks can work for 90-95% of applications where larger spacing between the machine feet and the mounting base is required; rather than inserting a big pile of shims, a chock can make life easier for the technicians performing realignment. 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.
This post was written in collaboration with www.machinerymountingsolutions.com