Torque Tips

Torque

Torque in rotational motion corresponds to force in linear motion. It is the product of the force tending to rotate an object, multiplied by the perpendicular radius arm through which the force acts. The net torque on an object is proportional to the resulting change in angular momentum. Torque is a vector directed along the rotational axis. If the fingers of the right hand curl in the direction of the change of rotation, the extended thumb points in the direction of the torque.

Moment of Inertia

A rotating body has the same tendency to maintain its state of rotational motion that a body moving in a straight line has to maintain its linear motion. The moment of inertia is a measure of a body's resistance to changes in rotation rate, analogous to mass as the measure of resistance to changes in translational motion. Specifically, torque T and angular acceleration (designated by the Greek lower-case letter alpha) are related through the moment of inertia I by the equation T = I(alpha) , just as force f and acceleration a are related through the mass m by the equation f = ma .

The moment of inertia depends not only on the mass of the body, but also on the distribution of mass relative to the axis. This distribution accounts for the fact that objects of various shapes with the same masses and diameters (such as sphere, solid cylinder, hollow cylinder, or wheel and axle) will not take the same time to roll down an inclined plane. Objects whose mass is concentrated near the axis have the smallest moment of inertia and thus, reach the bottom of the plane sooner than the others.

General

During the 1960's the U.S. Air Force conducted a study in order to determine the effect that improper torque practices had on peace time military air crashes. The results of this study were shocking. The data showed that at least one aircraft of every type used by the airforce had experienced a crash due to improperly applied torque applied to a critical fastner. This study helped redefine the importance of torquing specifications and the importance of torque wrench use, maintenance and calibrations. In another industry, an Indy racecar driver in one year experienced several half million dollar engine failures. Some of these failures were determined to be caused by the improper torquing of fastners. It should be noted that the mechanics in their haste to meet deadlines were mishandling these wrenches by throwing them into their tool boxes when they were done using them. The owner of the car and the driver soon realized the importance of proper maintenance and care of these tools and contacted a manufacturer and placed an order for all new wrenches. The difference with these wrenches was the gold plating and felt lined storage boxes he requested. This was done to give the impression of quality and importance to these tools.

Why do I mention these extreme failures? To stress the importance of maintaining all of your torque measuring instruments. Below I will list some things that you can due to help maintain your tools at peak performance.

Causes For Failures In Torque Wrenches

Use: The more often a wrench is used the faster it will wear.

Misuse: Any measuring instrument that is used for any purpose other than what it was designed for usually will experience premature failure or become suspect in its accuracy (i.e. dropping or using it as a pry bar or a hammer).

Improper Maintenance: Failure to inspect, lubricate, adjust, replace worn parts and failure to recertify to a known traceable standard causes the instrument to become suspect.

Improper Storage: Storing any instrument in a location that has high temperatures, humidity or in a corrosive atmosphere may cause failures. Storing adjustable click style wrenches above the lowest setting will cause premature main spring wear.

Torque Wrench Care

• Wrenches should be stored in cases if equipped. Do not store in areas of high temperatures, humidity or in areas where corrosive vapors are present.
• Adjustable "Click Style" wrenches should be stored set to the lowest setting on the scale.
• Ratchets should be inspected for wear and cleaned and lubricated often.
• Handle "Pivot Pins" should be inspected to ensure free movement of the handle.
• Beam style wrenches should never be etched on their beam or have labels placed on the beam.
• Keep moisture, paint, solvents from entering the inside of all wrenches. Wipe wrench often.
• Have wrench repaired as needed and recertified yearly at a minimum. Recertification should be determined by usage. Wrenches should be calibrated at 20, 40, 60, 80 and 100% of Full Scale or as instructed by the manufacturer. If a wrench is ever dropped it should be recertified prior to use.
• Wrenches should be calibrated and certified yearly at a maximum. Depending on usage, a torque wrench may require more frequent calibration. Selection of calibration standards or calibration vendor should be done with caution. Not all standards can certify all tools. The same is true of calibration labortories. Even though a vendor may be accredited the scope of accreditation should be read and understood. Many accredited laboratories have a limited scope of accreditation. These limitations may be due to accuracy, range technical competence, etc. If you are shopping for an accredited laboratory please compare our scope with a vendor that you are now using or are thinking of using.

   

   

Torque Wrench Use

Most torque wrenches are designed to be used at a specific length. Remember the formula for torque is:

Torque= Force x Length

Use of additional bars to the handles can effect the applied output torque and can cause damage to the wrench. Use of special adapters to the head of the wrench can be utilized but must be corrected for as they affect the torque seen by the fastner. If you are using a flat beam wrench ensure that the handle remains "floating" while applying torque. Click style wrenches normally have a line on the handle where force is to applied. Forces to all wrenches must be applied at 90° to the wrench.

Applying the force to the handle of the wrench should be done in a slow smooth manner. All torque wrenches are subject to Moment-of-Inertia errors. It should be noted that click style wrenches are highly affected by this.

Many wrenches are designed for use in one direction only if the wrench you own is one of these, do not use it in the opposite direction. Some manufactures will void the warranty if used to break loose fastners. If the wrench you own is designed for bi-directional use, excercise the wrench prior to using it when changing directions.

Follow all safety rules that the manufacturer and your company has provided you with.

I hope this information has helped you better maintain you torque wrench inventory. If you want further information please contact us at (847)838-1903 Fax: (847)838-2134 or e-mail us at: torque@jlwinstruments.com