Torque Wrench: Getting the Right Amount of Force

by | Torque Wrench

If leaving lug nuts too loose, it will creates the risk of slipping off while you are driving, which in turn could result in an unstable wheel or the wheel even coming off. However, if you snug them up too tightly, you might damage the metal in addition to struggling to get them unstuck the next time around. The trusty torque wrench is the perfect friend for you to tighten the fasteners without having to leave them very loose, tighten them too much or even destroy the metals involved.

In this article, I will guide you the basics of that define the features and functionality of the best torque wrench. The information in this article includes definition of torque value, reasons for the need of different torque values in tightening, and how to get the right amount of force with different types of torque wrenches.

Torque Value

Car and machine manufacturers specify the proper level of tightening, a torque value expressed in foot-pounds, for every fastener on your car. Torque is defined as a rotational force applied around a point or, in this case, a nut.

Torque is a measure of twisting force, typically expressed in ft-lb, lb-in, or N-m (foot-pound, pound-inch, newton-meter), that is force times distance.

To visualize this, put a 1-foot-long wrench on a nut and apply 10 pounds of force to the opposite end. You are now twisting that nut with 10 ft-lb (distance times force, or 1 foot times 10 pounds).

Use a 2-foot-long wrench and apply 50 pounds of force, and you will have 100 ft-lb, which is almost what a good number of lugs wrenches and is also force that is almost that which most elbows are capable to crank on.

While most mechanics rely on a well-calibrated elbow for tightening up things, it’s vitally important that the tightness these fasteners are subjected to should fall within some fairly narrow gap or range.

If it is too loose, there is the danger of the nut or bolt spontaneously unscrewing down the road. Or maybe the O-ring or the gasket that’s clamped by the bolt may leak.

If it is too tight, there will be other risks: The bolted-together part may be compressed, bent or otherwise damaged. The bolt shank could be broken, or the threads may be stripped, providing no clamping force at all.

Therefore, the best way to tighten fasteners is with a device called a torque wrench.

Why Do We Need Different Torque Values in Tightening?

Why don’t we just tighten every fastener of any particular size to the same torque value? Why do we need a shop manual to tell us that a 5/16 inch nut or bolt that holds down a valve cover will need an 11ft-lb force, while the 4⁄16-inch stud on the shock absorbers requires a 20 ft-lb force?

To answer these concerns, let’s discuss what happens when you turn a nut or bolt head.

The threads are twisted inclined planes or wedges, the simplest type of tool. As the inclined plane is wedged (turned) into the threads, a force is applied along the length of the bolts thereby making the bolt to be a special tension spring. This tension in the bolt shank clamps two parts together.

If the clamping force is greater than the load exerted between the head and the block, those two pieces will never spontaneously get loose. It is also worth mentioning that the more twisting force you apply to the head of the nut or the bolt, the more the clamping force exerted in the joint. So just tighten­ it until it will not come loose, right?

You are wrong. Differences in overall bolt length, the material of the clamped parts, the presence of a gasket between the two parts, and even the alloy of the bolt itself affect the proper torque, so that means that the clamping force can vary widely.

How to Get the Right Amount of Force with Different Types of Torque Wrenches

Depending on the application, there are many torque wrench types available, but two of them are most-common in the automotive field, including bending-beam torque wrenches and micrometer “clicker”. Each type works slightly differently to accurately measure how much twisting force you apply to a given fastener.

Bending-Beam Torque Wrench

This refers to the wrench for most of those who do not frequently need a torque wrench. The instrument has a large center beam that bends as you apply torque, while the unbending pointer beam, lets you read the torque directly. Once the inputted amount of torque has been applied, there is a release mechanism in the wrench’s handle that will stop any additional force from being put forth, the scale moves. On the indicator, you can see when the required toque has been reached. In case it goes beyond the calibration, you simply need to bend the pointer back to point zero using a pair of pliers.

The single biggest disadvantage of this kind of wrench is that your eyeball has to be directly parked above the pointer while you read the scale. This is quite difficult for the case of places that are hard to reach.

Micrometer “Clicker”

This pro-grade tool is preset to the correct torque having the feature of audibly and tactfully clicking on reaching the correct torque. It is highly repeatable and accurate, but should always be returned to point zero after every use. Even so, it ought to regularly be calibrated if used for critical parts such as internal engine fasteners as well as suspensions. Don’t use your torque wrench like a ratchet for any work such as disassembly, save it only for the final assembly.

The scale is located on the wrench’s shaft area will flash the torque units. Rotating the handle will set the torque to the desired level. For instance, if you wish to set the torque level at 26 feet pounds you will have to rotate till the marker is between 20 and 30 on the shaft. Then make sure that the marker for 6 units on the handle is in line with that on the shaft. Turn the wrench smoothly in the clockwise direction tightens a spring inside. When the torque reaches the estimated value, a signal will automatically release the handle, the tool clicks when you apply torque to a nut or bolt.

To Lubricate or Not

Most times, the specified torque values always assume dry and very clean parts. Clean means no dirt, rust, dried-up ­gasket sealer or any other form of dirt except just the shiny metal. Wire-brushing the threads will help you remove rust or sealant. Engine fasteners such as head bolts or main cap bolts are often specified to be worked on or torques with 30-weight engine oil wetting the washer and the threads. You will need to reduce the torque by between 15% and 25% if you are working on a fastener that has dry torque specifications and your threads are oiled. This is because the slipperier surfaces will decrease friction. If you fail to heed this advice and the fastener will be seriously over-tightened, you will either snap it or crush a gasket till it leaks.

On the other hand, rust or burrs on the threads can increase friction enough that a fastener tightened to the specified value won’t provide sufficient clamping force. The shop manual will specify whether the fastener is supposed to be dry or lubed. In either case, prep your bolts. If you’re really fastidious, clean up with some aerosol carb or brake cleaner, followed by more air. If you’ve used grease or anti-seize compound to keep the brake discs from seizing to the hubs, take care not to contaminate the studs or lug nuts.

Torque-Plus-Angle

Torque plus angle tightening is highly needed when fastening increasingly critical bolts, nuts or fasteners such as the engine-mount fasteners, cylinder-head bolts as well as the intake plenum bolts. Due to inconsistencies in the friction between the threads and bolt face, it is inconvenient to consistently torque a fastener. A few percentage points of difference in friction, as a result of thread irregularities in the threads, a burr, rust or old thread sealer, may result in to a huge variation in the fastener’s tension. That can lead to spontaneous dismantling of the joint subjected to the torque.

Critical fasteners are so difficult to properly tighten and fasten. It is because of this that the manufactures always ensure that they have specified the torque-plus-angle of tightening. The bolt is a one-time-use item that has to be discarded if it is ever removed. When you are fastening them, they are always stretched to almost their elastic limit. If you unscrewed the bolt at this point, it would return to its original length. Apply the extra rotation and the bolt stretches to its plastic deformation region, where additional stretching of the bolt yields no additional clamping force but the bolt won’t return to its original length later.

It is very important that for you to achieve the best torque results as far as tightening of bolts and nuts is concerned, you need to employ the use of a torque wrench. This will help you to avoid many eventualities such as spontaneous loosening of bolts and nuts when the car is on the move.

Do you finish torquing the wheels or ­cylinder head? Don’t forget to save the torque wrenches’ calibration by returning the micrometer scale to zero and store it in your tool box!

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