Few maintainers have received even basic training on fasteners and how they function. There are a lot of misconceptions about how fasteners actually achieve the required clamping force, through the material elasticity in the threads, with which to hold equipment together, the effectiveness of lock-washers, the roll of lubricants, etc. And, sadly, torque wrenches are a rare sight in the maintainer’s tool box. Here are some common mistakes seen on the plant floor related to fastener management:
Lack of torqueing standards in common maintenance work instructions.
Simply stated, we must provide clear instructions about the size and type of nuts, bolts and washers to be used. We must also specify required torque value, lubrication instructions, bolt sequence and any special instructions for tightening down equipment. Take a survey of your work instructions to determine if you’re providing clear instructions to your maintainers.
Use of undersized fasteners.
This is frequently seen on conveyer systems. Undersized nuts and bolts lack the required surface area around the slots to achieve the required clamping force to secure the equipment. A common sight is flat washers that are bent down into the slot. When this occurs, your flat washer is functioning as a spring and significantly reducing the clamping force, which in turn allows the fasteners to vibrate loose.
Loose or missing fasteners.
This is a tell-tale sign that the organization does not conduct routine fastener audits to verify the presence, proper condition and proper torque value of fasteners in place. These audits should be routine proactive PMs performed at least once yearly – more frequent in aggressive vibratory or environmental conditions.
No torque wrenches.
As previously noted, torque wrenches are a rare site on the plant floor. Mechanics should be issued an appropriate set of standard torque wrenches for day-to-day use and specialty wrenches (such as high torque hydraulic or pneumatic wrenches) must be available. All torque wrenches must be properly maintained. In addition to fastening during assembly, torque wrenches are required for routine torque audits and tightening PMs. Specialty torque measurement devices are also available and very useful.
Improper foundation and shimming.
Shims are designed to correct for incongruities between the mating surface of the machine and the foundation. In some cases machines are shimmed with material that’s too compressible and in other cases, shims are simply not applied properly. In other cases, the mating surfaces are simply to “untrue” to shim, usually indicating problems with the foundation. Another problem can be the foundation to which the equipment is attached itself, which simply allows too much movement. For example, overhung equipment, when it’s mounted to a flimsy structure will vibrate. Sometimes this is intentional, when the designer wants forces to relive to the structure with flexibility. In other instances, it’s just poor design. Poorly laid concrete foundations, for example may have too much undulation, poorly mixed concrete or improperly secured bolts.