Tom Bishop, P.E. 
EASA Technical Support Specialist 

We’ll begin by clarifying what is meant by a “soleplate.” In this article, the soleplate is the steel plate or baseplate on which an electric motor or similar rotating machine is mounted. In addition to issues related to soleplate condition assessment and restoration, the article will also cover other areas related to the concrete foundation upon which the machine and soleplate are mounted.

The following is a checklist of problem items to look for when inspecting a soleplate and foundation: 

• Settling of base soils 
• Soleplate deteriorating from rust 
• Cracks in concrete base or concrete joints 
• Evidence of concrete or grout shrinkage 
• Condition of grout bond 
• Distorted soleplate condition 
• Pipe strain 
• Loose mounting bolts 
• Loose or sheared dowel pins 
• Loose shim packs 
• Paint on shims 
• Rusty shims

We will address these and other key issues related to soleplate integrity. 

Note: “Tips For Proper Baseplate Construction” in the April 2001 issue of CURRENTS addressed baseplates, focusing on service center test beds. 

Soleplate basics 
Basic requirements for solepates include the following: flat mounting surfaces, and a stable and rigid base. (See Figure 1.) Although there are no industry standards for flatness of motor feet, at least one standard requires foot flatness to be within 0.005”. Prefer­ably, the soleplate should be flat within 0.002”. A stable and rigid base will not distort under load conditions, or due to such stresses as welding or differential expansion of different machine components. To achieve this rigidity, the soleplate may need to be stiffened with gussets or braces. If the soleplate stiffness is not adequate, the machines mounted on it can become misaligned, and vibration levels will increase due to distortion (twisting) of the soleplate. 

If the equipment soleplate and foundation are relatively massive, vibration issues related to natural frequencies (resonance) are rare. However, the possibility of resonance increases with motors operated from VFDs as the drive assembly natural frequency may coincide with operating fre­quency at certain operating speeds. VFDs often have the ability to block out problem speed ranges.

When the soleplate and foundation are not massive, the possibility of a resonance condition is greater. Some simple approaches to deal with resonance are to add weight to the foundation or equipment to lower the natural fre­quency; or stiffen the structure to increase natural frequency. 

Soleplate installation 
Although few of us deal with completely new installations, it is useful to know how the original soleplate should have been installed. This knowledge can be helpful in determining if the original installation was proper, or to identify deficiencies in the original construction. 

When a soleplate is installed on a foundation it should be mounted using shims and grout. The founda­tion should be a permanent rigid installation of concrete or other material of sufficient mass to absorb normal vibrations. Check for inten­tional vibration joints or air gaps between the machinery foundation and surrounding building structure; these help to prevent transmitting vibration.

Check the foundation for evidence of any settling of the underlying base soils. The soleplate should be painted to help prevent deterioration due to corrosion (rust). Settling of the base soils and severe rusting of the soleplate are conditions that can reduce rigidity, leading to soleplate distortion and vibration. 

If concrete is being used, founda­tion bolts can be enclosed in a pipe sleeve two to three diameters larger than the bolts to compensate for minor variations in alignment. Check for evidence of cracks in the concrete base or in concrete joints. (See Figure 2.) Vibration can cause, or may be caused by, cracks in the concrete base or joints. Also check for evidence of concrete or grout shrinkage. Such shrinkage can result in foundation looseness. 

Proper grout material should not shrink. Looseness caused by concrete shrinkage may be repairable by filling in with epoxy grout material. If there is evidence of grout shrinkage, consider replacing all of the grout. Also, check the condition of the grout bond to the soleplate and foundation. If there has been grout shrinkage, the bond may not be complete. 

Soleplates should be grouted to the foundation if pos­sible. Completely filling under the soleplate will provide a more stable installa­tion. Grout performs a number of functions including acting as a stiffener to help avoid resonance and as a vibration dampener by placing the rigid mass of the founda­tion in contact with the soleplate. It also anchors the soleplate to the founda­tion so the soleplate and equipment do not shift. 

A word of caution: The soleplate should not be grouted to the founda­tion until after the equipment has been final-aligned. Coarse alignment is usually done before grouting, and final alignment afterwards. Following grouting the alignment should be rechecked, and the final alignment made after the motor and driven equipment are at normal operating temperatures. 

Soleplate condition 
The ideal mounting arrangement for equipment on a soleplate is to have the equipment feet flat and the soleplate mounting surface flat; that is, all in the same plane when the machine is secured to the soleplate. In reality, perfect flatness is not achiev­able. However, the preferred values for flatness of motor feet and soleplates are 0.005” and 0.002”, respectively. Gross distortion of the soleplate can be checked with a straight edge. Laser tooling can be used for precise measurement of soleplate flatness. If the soleplate is distorted it should be removed, if possible, and machined flat. As an alternative, if the soleplate cannot be easily removed, a portable mill can be used to machine the soleplate on site. 

The soleplate should extend beyond the outer sides of the mounted equipment feet. Check that the soleplate extends beyond the equip­ment feet, and also check the mount­ing bolts. Some installers may use reduced shank (“Chicago’d”) bolts due to the original bolts being bolt-bound. The associated equipment holes should be machined oversize to accommodate the proper bolt size. 

Also check that the soleplate is level. If a sleeve bearing machine is mounted on a soleplate that is not level, thrust load may be imposed on the bearings, resulting in higher bearing temperatures and premature bearing failure. Most electric motor sleeve bearings can only accommo­date accidental or very short-term thrust loads. 

Alignment 
Jackscrews make alignment easier by providing a means to shift larger machines axially and laterally on the soleplate. If the soleplate has jack-screws for adjusting the location of the motor and driven equipment, check for distortion to the soleplate possibly caused by over tensioning of the jackscrews. Likewise, check for damage to the jackscrews due to over tensioning or physical damage. 

Distortion of the soleplate may be identified by alignment checks. Before a motor is removed, check operating (“hot”) alignment. If the motor is already higher than driven equipment, and flat on the soleplate, proper alignment may not be possible. The solution is either to raise the driven equipment, or to mill the feet of the motor to allow at least 1/8” (3 mm) of shimming, if possible.

Caution: If the amount to be milled off the feet will require milling of the bottom of the frame, do not proceed. An alternate method should be used, such as raising the driven equipment. Also, check for soft feet; a value greater than 0.002” indicates need for correction. (See Figure 3.) 

The soft foot check can be done using laser alignment equipment or with a dial indicator. To use the dial indicator method, apply the indicator to the top of a mounting foot, then loosen the foot bolt and read the change in indicator reading. Tighten each bolt after measuring the deflection (if any) and proceed to the next bolt until all bolts on the motor and driven equip­ment are checked. 

If a disbonded soleplate is suspected, compare vertical vibration readings on all mounting feet and various positions on the soleplate itself. Logic helps here: The direction of shaft rotation is a good indication which side of the soleplate is most likely to become loose in service. Facing the drive end of the motor, if the rotation is counterclockwise, the operating/starting torque applies force to lift the left side of the soleplate. If the vertical vibration is higher in that area, the grout bond between the soleplate and foundation is loose. The soleplate should be removed and re-grouted. 

Pipe strain 
Driven equipment such as a pump connected to piping can be distorted by pipe strain, which in severe cases may affect the soleplate as well. To check for pipe strain, loosen the flange bolts and make sure the bolts are not bound by lateral pressure between the flanges. You should be able to move the flange bolts by hand once the bolts/nuts are loosened. The driven equipment should not be used to support piping. To avoid misalign­ment and distortion, check that other means such as pipe hangers and pipe supports are used to carry piping. If any changes are made topiping or pipe supports, it is advisable to recheck the alignment with the machine and piping at normal operating temperatures. 

If the installation is new and the as-found parallel or angular mis­alignment is great, this is an indication of soleplate distortion. The distortion could have been caused by stresses during shipping or installation. Check the soleplate for distortion using laser alignment equipment. After approximately two weeks of operation, check alignment again to make certain that tempera­ture changes, piping strain, or foundation variations have not caused misalignment. If alignment has been maintained, the driven equipment and motor should then be doweled to the soleplate. 

Doweling and shimming 
Doweling maintains alignment by preventing equipment from shifting on the soleplate, even if vibration may loosen some of the fasteners used to connect the motor and driven equip­ment to the soleplate. Check the mounting bolts for tightness; and even if tight, also check that the dowel pins are tight and not sheared. 

Stainless steel pre-made shims are available from a number of suppliers. These are resistant to corrosion and do not have the sharp edges that often result with hand-cut shims; many have their thickness dimension permanently marked on them to facilitate selection of the desired thickness. The shims should extend at least 1/4” (5 mm) beyond the equip­ment feet they are used under. Check that the shims extend beyond the equipment feet. Also check for loose shim packs, paint on the shims and rusty or deteriorated shims. Any of these conditions can reduce rigidity and lead to soleplate distortion and vibration. 

The shims should be of an appropriate size for the feet (i.e., they should support the entire foot) and should support the vertical structural portions of the frame. In other words, the shim should surround the base bolt and must also be directly beneath the frame stiffeners. 

Categories: Base/soleplate/foundation/mounting

Tags: Baseplate

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