Torque

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maintenance

June 11, 2007

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NEWSLET T  TE   R FOR MAINT ENANCE COMMUNIT Y

Word for the day: TORQUE

In the Indian sub-continent, we are always fascinated by the ability of “Royal Enfield” (popularly named “Bullet”) to travel longer distances and carry “heavy” load without any problem. Royal Enfield generally comes in two variants, 350 and 535 CC engine capacities with top speeds of more than 100 kilometers per hour. One of the “hidden” parameters of this engine and for that matter, any so-called “powerful” machines is Torque. From every day simple application of locking the doors, opening bottle cap to carrying 40 feet fully loaded container, “torque” is an integral part of the activity. Torque is defined as the rotational force caused by applying certain amount of force at a distance. Refer figure 1 for example. Here the  torque applied on a nut is decided by the length (r) of adjustable spanner (wrench) and force (F) applied at the end. Remember that  torque always causes the object to”rotate” and hence sometimes it is also referred to as “turning moment.” In this example,  torque, T = r x F = distance from center x Force applied. For instance, if r is equal to 1 meter and F is equal to 10 kilograms,  torque applied on the “nut” becomes, T = 1 x 10 = 10 Kilogram-meter (Kg-m) = 10 x 10 = 100 Newton-meter (Nm). We can obtain the same torque by applying 5 kilograms at 2 meters (T = 2 x 5 = 10 Kg-m). The “rate” (speed) at which torque is applied is called “Power” and plays a vital role in selecting proper machine Figure 1 for an application. Torque is also very important in defining capacity of various automobiles. To understand, how they are interrelated, let us take one 3 phase induction motor as an example and study various parameters. Motor power, P (Kw) = 22 Motor Speed, N (RPM) = 1440. Following formula can be used to calculate the torque.

22 = Torque (Newton-meter) x π x 1440 / 30000 Therefore, Torque (Nm) = 22 x 30000 / (1440 x π) = 146 Nm = 14.6 Kg-m From this formula, we can see that as speed reduces, torque increases and this concept is used in reduction gear boxes, automobile gear boxes to modify the torque. Let us now use the motor to perform a useful task. A one meter diameter pulley is expected to lift 2000 kilograms of weight in an over head crane. Torque required, to lift the weight, T = 2000 x 0.5 (we have to consider only the radius) = 1000 Kg-m = 10000 Nm But since the selected motor can generate only 14.6 Kg-m, it cannot directly lift the weight and hence speed needs to be reduced and hence we will install a gear box to generate the required torque. Let us now calculate the gear ratio. Required Torque, T2 = 1000 Kg-m; Available Torque = 14.6 Kg-m Therefore, required gear ratio = T2 / T1 = 1000 / 14.6 = 68.5. With safety factor we can round this figure off to 70. Since gear ratio is 70, the rpm of pulley gets reduced by same ratio and will now be equal to (1440 / 70) = 20. If the designer finds this rpm too low, then kilowatt (horsepower) rating of the motor gets increased. In this application, if  gear ratio or motor capacity is reduced, it cannot lift the weight and motor gets either burnt or overloaded. Torque is also vital parameter in bolts which are designed to hold mechanical assemblies together. Normally the “tightening torque” is indicated in foot-pounds for most of applications. For many assemblies, even though hand If you like to improvise this article or contribute or comment please mail us at: [email protected] This document contains information for reference only. We assume no responsibility for its implication.

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June 11, 2007

tightening is an accepted practice, torque wrenches have to be used in very important applications, like engine head, clutch assembly, flywheel assembly and more. On torque wrenches, the user will adjust prescribed value and start tightening the bolt. As soon as set torque is reached, torque wrench clicks and does not tighten the bolt any further. This will prevent major “error of judgment” of hand tightening which needs to be avoided as much as possible, at least in critical assemblies. Let us take a brief look at commonly used units for specifying torques. Base Unit 1 Kg-m 1 N-m 1 Lb-f 1 Lb-In

Kilogram-meter (Kg-m) 1 0.1 0.139 0.115

Newton-meter (N-m) 10 1 1.39 1.15

Foot-Pound (Lb-f) 7.24 0.74 1 0.084

Inch-Pound (Lb-In) 86.8 8.88 12 1

With increase in complexity of production facility, it is essential to prevent machine breakdowns as much as possible. TPM – Total Productive Maintenance – a concept conceived by Japanese is becoming extremely popular in reducing breakdowns and increasing efficiencies of machineries. As per 40 year long analyses made by TPM, it is observed that “improper tightening” is one of the important causes of major breakdowns. In other words, applying less than or more than required torque (usually to prove one’s physical strength in shop floor!) is the root cause for such breakdowns. Some of the points to be considered with reference to torque are as follows  

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Higher the weight to be lifted (crane for example) or moved (automobile), higher the torque necessary If an undersized prime mover (motor, for example) is selected, torque can be increased by reducing the gear boxes (speed) If both high torque and high speed are essential, the prime mover’s size will increase considerably. When tightening bolts, its torque limits should not be exceeded Torque wrenches should be made a standard part of maintenance department tool box

If you like to improvise this article or contribute or comment please mail us at: [email protected] This document contains information for reference only. We assume no responsibility for its implication.