When even one foot of a gearbox differs in height from the others, you’ve got a case of soft foot on your hands. Soft foot occurs when the machine’s casing is not in proper contact with its baseplate. Unless it is dealt with, this condition can cause misalignment, looseness, vibration and internal clearance issues within a gearbox – and it can add extra pressure on any Falk coupling to which the drive is attached.
Identifying Soft Foot
There are two types of soft foot: angular or parallel. With a parallel soft foot, the foot remains parallel to the base but lacks contact. An angular soft foot may make contact with the baseplate, but the contact is not uniform; when the base bolts are tightened, the foot will bend to conform rather than rest securely and evenly on the base. A case of soft foot may be either angular or parallel, but most are a combination of the two.
Causes of soft foot in a gear reducer can include:
- Warped, dented or otherwise damaged foundation, baseplate or feet.
- Wrong amount of shims under the feet.
- Dirt, a spongy layer of oil or other items between the feet and base.
- An attachment (such as a pipe) that prevents the machine from fully resting on its base.
- Excessive tension on the feet caused by jack bolts.
A hallmark of soft foot is that when you tighten the bolt where it is located, the relative position of the shaft will change. When using laser alignment tools, you may find that as you tighten a particular bolt, that foot causes a clear change in the vertical or horizontal readings.
Because soft foot can lead to misalignment, it’s common to assume that the misalignment itself is the problem without further investigating the root cause. One way to determine whether soft foot is the culprit is to loosen each foot bolt in turn, leaving the others tight. Normally, loosening a single bolt will increase vibration amplitude; if it has the opposite effect, you’ve got soft foot.
To differentiate soft foot from looseness, take phase measurements between the foot and the base. A difference of 180 degrees points to looseness.
Minimizing Soft Foot
Soft foot can not only cause alignment problems with your Falk coupling, but can take its toll on the entire system. Here are some tips for controlling it:
- Ensure that the baseplate is installed and leveled to the manufacturer’s specifications.
- Check that the baseplate and feet are deburred, clean and dent-free around the mounting areas.
- After checking the reducer thoroughly for soft foot, correct any cases using the minimum amount of shims necessary. Use only shims that are flat, clean and deburred.
- Tighten foot bolts in 3 passes (first hand tighten, then wrench to about 50 percent, and fully tighten on the third pass) using the same bolt torque pattern each time.
By watching for and carefully controlling soft foot, you can extend the life of your Falk coupling – and any other connected equipment.
Click here for more information on Falk Couplings.