Reprinted from i Saluti, February 1998

Up Tight With Torque Wrenches

by Charlie Zarek

The recent articles, "Threaded Fastener Facts" by Andy Pugh, appearing in i Saluti prompts me to share a few bits of information about torque wrenches.

The torque wrench as it is know today was a development in the mid 1930’s of the Chrysler Corporation and a company known as Micromatic Hone. It was first manufactured and sold by Cedar Rapids Engineering Company. P. A. Sturtevant was the first individual to sell torque wrenches to the trade.

Of the several type torque wrenches. The following three are most common in the auto shop environment: The direct-reading bending beam torque wrench. The rigid frame toggle action wrench. And a third, the incased beam dial-indicating torque wrench. There are others that are out of the scope of this article.

The direct-reading bending beam type is the most rugged. While other types are affected by wear and fatigue, this tool will remain accurate so long as the beam is not actually broken and the pointer is set to zero. I use one regularly that is well over 30 years old. The disadvantages of the bending beam configuration are:

The operation of the direct-reading bending beam wrench is rather straight forward. When torque is applied to the fastener, the beam bends a known amount but the pointer stays straight indicating directly on the scale, how much torque is being applied to the fastener. Simple as that.

The handle of such a torque wrench is mounted on a pivot which concentrates applied force to a specific point on the beam to ensure greatest accuracy--see Fig. 1.

The toggle action torque wrench has now taken the predominate place in the auto shop due mainly to the availability of inexpensive tools of this type and overall, ease of use. The toggle action is set to a predetermined value by the operator. When used for its intended purpose a click is heard and felt when the set value is reached. Just grab the handle, pull, click.

The toggle action wrench has a coil spring measuring element built within a rigid housing. Positions of the adjustment screw impose different amounts of spring pressure to a toggling mechanism within the housing. This spring pressure holds the end surfaces of the load lever, toggle, and the toggle slide bar tightly together. As force is applied to the handle, the fastener is turned until it's resistance to the turning force overcomes the pressure of the spring element. At this point the end surfaces of the load lever, toggle, and the toggle slide bar break apart or toggle. The wrench now becomes rigid as the load lever is resting against the inside surface of the frame. The click is caused by the toggle action and the load lever striking the frame. To continue force on the handle after the click will apply torque to the fastener in excess of that set by the adjustment screw.

This tool has several advantages over other types:

While the toggle action wrench is subject to wear, the repeatability remains pretty consistent during a use session although the dialed in accuracy may degrade with improper care. The action of the wrench is depicted in Fig. 2.

The third type addressed here is the incased beam dial indicator equipped torque wrench. This wrench has a stout beam for its measuring element. The beam is attached to a rotatable drive head and rests against stops, which are part of the frame, at the other. When torque is applied to a fastener the beam is deflected by a small amount. The small amount of deflection is magnified mechanically and viewed on the dial indicator incremented to read torque in pound-feet or Newton-Meters--see Fig. 3.

Another type dial indicator equipped torque wrench has a torsion bar as it's measuring element. Due to the size limitations of hand held torque wrenches, the torsion bar must be very short. For this reason, the twist or angular movement is very limited. This type mechanism is rarely found in a hand held tool. I have not included an illustration for this type wrench.

The complex and delicate internal mechanisms of either the incased beam or the torsion bar tools discussed above are subject to wear, and friction losses. They don’t fare well in a dirty, greasy, environment, require regular calibration, and are relatively expensive. You are most likely to find dial indicating wrenches in high-end auto shops.

Adjustable or indicating torque wrenches should be accurate within 10 percent of the readings between 30 percent and 100 percent of full scale. Like any measuring instrument, it is best to avoid the extreme ends of the scale or range.

Torque Wrench Adapters

Adapters can be used at the drive end of a torque wrench. That is, should one need to torque a tubing fitting or a fastener that cannot be accessed directly on line with its axis. A crows-foot on a torque wrench is an example. Adapters can be fabricated from old tools or any of the cheap tools found in stores like (in the St. Louis area) Big Lots or Hoods.

Adapters can extend along the wrench’s centerline or extend out at 90 degrees from the drive end. Any angle between 0 and 90 degrees is possible. At angles other than 90 degrees, dimensions of the critical parts on the wrench as well as some simple math are required. Adapters extending at 90 degrees from the drive end induce no error in the scale reading--see Fig. 4.

Adapters should not be used on ratcheting type torque wrenches. The distance between torque centers can and does change as the wrench ratchets around the drive head. An exception is where the ratio of torque center distance (A) to the torque arm length (L) does not exceed 0.10. A crows-foot used to torque a tubing fitting will seldom exceed this ratio (A/L).


Handy tips that should be obvious:

While preparing this article, some questions arose as to why a handle extension could not be used on a rigid-frame wrench. My research material indicated that such a wrench a could be grasped anywhere along it’s length. So why not beyond it’s length? The instructions for the use and care of a Craftsman “Micro Tork” warn against the use of an extender. It is stated in bold type face; “Their use will result in erroneous torque readings, and may damage the grip or adjusting screw.” In my garage laboratory, I contrived a device to extend the handle which would not damage the grip or adjusting dial. Rudimentary tests did not prove the handle extender introduced error. I would follow the manufacturer’s recommendations in any case. I’m no physicist!

General Torque Specs for Metric Fasteners

Bolt  Material class (grade) in lb-ft
diam  "5.8" "8.8" "9.8" "10.9"
 5mm    3.5   5     6      8   
 6mm    6     9    10.5   12  
 8mm   15    22    25     32  
10mm   29    44    51     62  
12mm   51    76    89    111 
14mm   81   123   140    177 
16mm  125   199   214    280 
These torque values are approximate and should not be accepted as accurate limits. Indeterminant factors (surface finish, type of plating and lubrication) in specific applications preclude the publication of accurate values for universal use. Manufacturers of various types of equipment usually provide specific tightening instructions which should be followed. DO NOT use the above values for gasketed joints or joints of soft materials.

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