Kirk Kirkland
Electrical Repair Service Co.
Birmingham, Alabama
Technical Education Committee Member
Many progressive end users require validated precision in the manufacturing of new motors they purchase as well as with the motors they have serviced.
As repair service providers to these end users, EASA service centers are often required to comply with the standards as implemented by the original equipment manufacturer. This holds true for the most basic components of an electric motor: the bolts.
The basic motor assembly and the integral parts are normally secured via a specific type and grade of bolt. Therefore, validated bolt torque procedures and referenced bolt torque values are necessary to address general compliance issues.
Definition of torque
As generally defined, “torque is a twisting or wrenching effect, or moment, exerted by a force acting at a distance on a body, equal to the force multiplied by the perpendicular distance between the line of action of the force and the center of rotation at which it is exerted as commonly expressed in units of measure such as: inch pounds, inch ounces, foot pounds, Newton meters or Centi Newton meters.” See Figure 1. Therefore the objective is to have a procedure in place that addresses the compliance issue for bolt torque and resultant clamping force.
Controlling the torque applied in tightening threaded bolts is the most commonly used method for the application of clamping force. There are many factors that may affect the relationship between torque and clamping force of threaded bolts.
These include items such as lubricated threads, the material from which the bolt and receiver are made, the type of washers used, the class and finish of threads and various other factors.
It is impossible to establish a definite relationship between torque and applicable clamping force for all conditions, situations and specialty requirements. In general, a minimal part of the torque applied to a bolt contributes to clamping force. The remainder, which can be as much as 90% of the total applied torque, is used to overcome friction under the bolt head (or between washer and receiver) and friction in the thread engagement.
Importance of torque
The importance of torque in many applications cannot be overemphasized. There are critical applications in which the bolt torque is very sensitive to the manufacturer’s design. Be aware of the special design situations such as vibrators, shakers, crane/hoist applications, etc. and contact the manufacturer for details.
Under-torque can result in unnecessary wear of components and may contribute to premature mechanical failure. When insufficient clamping forces are applied, uneven loads will be transmitted throughout the assembly, resulting in the potential for excessive or premature wear and fatigue failure. Over-torque can overstress the bolt, bolt receiver and loading of the clamp area. Torque requirements should be determined by bolt grade, size, thread conditions and the associated clamping necessary to secure the components.
The mathematic relationship between torque applied and the resulting force in the bolt is the formula
T = K x d x P
as defined by the following:
T = Torque required (inch pounds,Newton meters, etc.)
K = Coefficient of friction factor
d = Nominal thread diameter (inches, millimeters)
P = Tension (clamp-load or pre-load) induced in the bolt (pounds, kilograms)
Most of the torque/tension tables that have been developed and published over the years utilize this formula.
The value of “K” can range from0.30 for a rusted assembly to 0.10,or less, for a clean well-lubricated assembly using a proven thread lubricant. Make note that the “K” factor is subject to application variables; the industry-accepted “K” factor for clean plain bolts is 0.20. However this factor is reduced to 0.10 should the bolt be well lubricated which will result in an entirely different torque figure.
Table 1 below lists generally accepted figures for the “K” factor based on industry standards.
A commonly asked question is: “What torque should be used to tighten Grade 5 bolts?”Table 2 considers the above-listed torque formula and ASTM standards. Notice the dramatic reduction in torque applicable to lubricated threads.
Note that the torque values suggested in the table above are presented as a general guide only. Torque is a very indirect indication of tension; there might be other factors involved such as friction, lubrication, surface texture, rust, material type, the use of washers or prevailing-torque nuts, thread conditions, debris, etc. that may affect the outcome.
Charts for other types and grades
There are additional bolt torque charts available for other bolt grades and types; please be sure you have the appropriate chart. A properly tightened bolt is one that is stretched so it pulls mating surfaces without exceeding the design rating of the bolt or components.
In summary, the torque applied is simply a measure of the amount of energy required to spin the bolt along
the threads, whereas tension is related to the stretch or elongation of the bolt that provides the clamping force of a joint.
The following are quick reference tips for bolt tightening:
- Know the bolt grade and type to be used.
- Reference a bolt torque table for target values.
- Determine the torque target value and special considerations such as whether you are working with dry or lubricated threads. Lubricated threads call for a lesser torque target value than dry threads.Torquing a lubricated bolt to a dry torque value may result in the failure of the bolt or bolt receiver.
- Use a calibrated torque wrench for tightening only. The calibrated torque wrench should be properly sized for the bolt to be tightened. Do not use a “cheater” (extension) bar as this will invalidate the torque reading resulting in non- compliant bolting procedure.
- Tighten the bolts in a balanced pattern when possible to reduce preloaded clamping of the joints to be secured.
Conclusion
Implementing proper bolt torque procedures is a quality issue that reliability and precision-oriented customers are demanding. There is a culture change that may need to take place on the service center floor. This requires a move from using an impact wrench or an extension bar to get the bolt as tight as possible, to the implementation of a bolt torque procedure. The ultimate objective is that the motor has been assembled to a specification that will add to the overall quality of the motor rebuild.
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