Machinery mounts reduce noise and increase productivity
Today's sophisticated equipment and machine tools have performance, repeatability, and precision designed in. These features cannot be realized without a proper installation specifically suited to that equipment's requirements.
Today's sophisticated equipment and machine tools have performance, repeatability, and precision designed in. These features cannot be realized without a proper installation specifically suited to that equipment's requirements. About 1% of a piece of equipment's value is required for proper installation. It is poor economy or ignorance not to give consideration to all facets of machine mounting.
While it may seem obvious, a good foundation is the basis for successful machine installation. There are many instances where the reasons for short equipment life, excessive maintenance, frequent realignment, and lost production were due to an installation that did not provide adequate support or isolation against shock and vibration and a means to attain and maintain an accurate degree of levelness.
Although manufacturers state their products meet OSHA standards, that is not hard to do. The OSHA handbook for general industry regulations contains this statement under CFR 1910.212, General requirements for all machines, Part b; Anchoring fixed machinery. Machines designed for a fixed location shall be securely anchored to prevent walking or moving.
Sources of vibration
Shock and vibration originate from nearby manufacturing operations and related activities in the plant. An often overlooked source of vibration problems is resonance of the floor. When mechanically excited, building floors can resonate at many frequencies which can cause equipment problems if not recognized and dealt with. Precision machine tools are more affected by frequencies higher than 20-24 Hz.
From an installation standpoint, equipment can be divided into those that are support-critical, such as precision machinery, and those that are not. The foundation is a primary factor in achieving optimum performance from support-critical machines (Fig. 1). Just because a machine is free-standing does not mean that proper support is not required. Basic concrete design procedures, as outlined in the table, should be followed. Some machines need to be bolted to a floor not because of support, but to add rigidity.
Since the foundation is an integral part of the machine structure it must be designed stiff enough to provide support. The basic foundation geometry is critical. It must also be designed to react uniformly to dynamic loads, no matter where they occur. Local soil conditions must be taken into account to ensure stability.
If total isolation is required, inertia block construction (Fig. 2) should be used, along with pneumatic isolators. This method suspends the machine/foundation system on deflecting elements that absorb energy. Transmission of shock and vibration can be reduced as much as 90%. However, isolated foundations are very stiff and do not offer any low-frequency vibration or shock isolation. These vibrations should be reduced before reaching a floor or foundation.
Once the foundation is determined, attachments or anchors should be selected to make the foundation and machine an integral unit. The anchor must have enough rigidity to meet machine requirements, yet be able to adjust the relationship of the machine base to the foundation.
Equipment that is not support-critical does not require external support to maintain critical alignments. A solid, flat floor and simple anchors may be all that is required. It may be necessary to prevent the machine from "walking" during operation. For some machine configurations an anchoring restraint could be required to eliminate safety hazards.
Leveling may also be a consideration if floor conditions are less than ideal. Levelers are available that provide for quick installation and equipment maneuverability.
Where anchoring is not required, a free-standing mount combining vibration isolation and leveling capability can be used. Such mounts have specially treated bearing pad surfaces to prevent walking.
For equipment that does not require leveling, simple pads provide efficient, low-cost vibration isolation while preventing walking. For installations where vibration isolation and anchoring are required, pads can be used with anchor bolts. This arrangement ensures shock/vibration isolation, while providing the positive restraint of an anchor bolt.
Equipment that generates high inertial forces relative to machine weight usually is installed using elastomer isolators. They are stiffer than spring isolators and control motion due to out-of-balance forces. Most machines are not dynamically balanced and generate impact, rocking, and inertial forces, causing them to move excessively if mounted on soft isolators, such as springs.
There are a variety of methods available to attach machinery to a floor. Some are simple to use, particularly when the equipment is self-supporting or must be moved occasionally. Others become very involved and provide rugged anchoring and precision leveling.
Felted fiber has been used for thousands of years to absorb shock and vibration. This material has been refined to be highly resistant or impervious to most industrial chemicals, oils, and moisture. In many cases its life expectancy exceeds that of the equipment used with it. Anchor bolts and isolating sleeves must be used with this material (Fig. 3). Load capacities exceed 250 psi.
An innovative product with limited application is a felt-like strip coiled in a can and impregnated with resin. The resin is a cold-setting, one-part compound that secures a machine to the floor in minutes. Bolts and grout are eliminated. The bond can be broken and equipment moved by applying steam or boiling water to the pad. A 4 X 4-in. pad can support 5000 lb. A level floor is required, unless the equipment has built-in leveling capabilities.
Neoprene pads with cast-in line or pocket patterns do not require tools, holes, bolts, or adhesives for installation. They are laid on the floor and the machinery is placed on them. Their design prevents machinery from walking or creeping. Custom fitting can be done with a knife or scissors. Solid neoprene pads, up to 1/2-in. thick, have a capacity of 300 psi. Pads laminated with a textile material, up to 1-in. thick, have a capacity of 2000 psi.
The next logical step in machine mounting is to combine leveling capability with the ease of installation provided by pads. A base with an attached elastomer easily slips under a machine foot. A screw is inserted from the top and used for leveling (Fig. 4). These devices can handle loads from 30-1,000,000 lb. Their load capacity depends on the amount of vibration and shock generated from the machine being supported.
This device permits fast, simplified, precise leveling. It is self-aligning due to the elastomer mounting and swivel arrangement which compensates for uneven floors. As with pads, no cementing or bolting to the floor is required.
Wedges, instead of direct-acting screws, are used to provide a leveling action under heavy machinery (Fig. 5). The load capacity of this type of mount ranges from 3000-250,000 lb.
While wedges can be used in plain steel form, an elastomer or felt pad can be applied to the top and bottom surfaces to provide vibration isolation. In addition, an aligning disc or seat can compensate for up to 5 deg of floor misalignment. These devices provide up to 1 in. of height adjustment and some versions accept anchor bolts.
Coil spring isolators are very effective in reducing impact and inertial forces (Fig. 6). They typically have a vertical natural frequency less than the machine's operating speed range. Due to their low natural frequency and softness, excessive motion can result if large out-of-balance forces exist.
There are two main types of motion that can occur with coil spring isolators: transient and steady state.
Transient motion occurs when a machine starts, stops, or runs through the isolator's natural frequency or resonance. One way to control this motion is to quickly accelerate through resonance.
Steady-state motion occurs at operating speed and is caused by out-of-balance forces generated by the machine. This force can be excessive if the vertical inertial force exceeds 10% of the machine's weight.
Proper damping in coil spring isolators is essential in controlling transient and steady-state machine motion. Damping is most effective in controlling transient motion. The type of damping used depends on the application. Too much damping can increase the amount of force transmitted to the foundation, and too little can result in excessive machine motion.
Anchor bolt combinations
Anchor bolts can be used with shims, leveling screws, and wedges. The choice depends on how precise leveling must be and how rigid a mounting is required.
Anchor bolts and shims offer an inexpensive and moderately rigid connection between machine and foundation (Fig. 7). This design may not be adequate for high precision machinery because the large number of shim interfaces under load compromises rigidity. Another drawback is precise alignments are often difficult to achieve.
Anchor bolts and leveling screws permit faster adjustment than using shims (Fig. 8). However, more frequent alignment is usually necessary. Precise alignment is difficult.
Anchor bolts and wedges provide a firm and stiff support (Fig. 9). The basic leveling wedge is a two-piece device that provides vertical adjustment by moving one wedge against the other with an adjusting screw. An effective design uses a three-piece wedge. The top wedge has no horizontal movement trying to move the equipment while an adjustment is being made.
Mount systems are available which combine a three-piece wedge and spherical seat arrangement in the upper wedge. This arrangement compensates for misalignment between the floor surface and the machine base. In some systems alignment can be accomplished with anchor bolts already tightened, simplifying the process.
Plant Engineering magazine extends its appreciation to Barry Controls, Unisorb, and Vibro/Dynamics Corp. for their assistance in the preparation of this article. The cover picture was furnished by Barry Controls.
--Joseph L. Foszcz, Senior Editor,
Good mounting practices extend machine life, reduce maintenance, and increase uptime.
Sources of vibration problems can come from equipment both in and out of the plant, as well as the plant floor.
The type of machine mounting selected depends on whether the equipment is support-critical or not.
Typical vibration sources
Machine tools making
Inertia block foundation checkpoints
- Design for allowable deflection, not allowable load. The overall foundation stiffness for nonsupport-critical machines must provide for the maximum performance of any free-standing mounts used.
- In most cases a mass ratio of foundation to machine of 1.5:1 for support-critical machinery and 1:1 for nonsupport-critical machinery is desirable.
- The foundation is a support only for the machine to be isolated. All air, water, and electrical connections should be flexible.
- The combined center of gravity of the machine-foundation should be below the top of the foundation. It should pass through the center of the soil pressure diagram within 5% of the smallest horizontal dimension in any direction.
- Verify allowable soil loadings. Reduce soil load capacity by 50% when the equipment generates significant dynamic loading.
- Run a vibration spectrum analysis under conditions duplicating operating conditions for vibration sensitive equipment. A below-grade soil survey provides an indication of the lower end of the ambient vibration spectrum.
- Allowable concrete and steel loads should comply with applicable building codes. Appropriate fatigue factors should be used when high amplitudes will be present.
- Anchor bolt locations should be at least 12-in. from the foundation perimeter or a distance equal to anchor bolt embedment depth, unless reinforcement is provided in vertical walls.
Since there is a notable lack of material available on machine mounting, the best sources of information are the manufacturers listed in the table at the end of this article. They can provide engineering information and recommend the correct product to use in an application.