Power at Work: Back to Basics - Part III: All you ever wanted to know about torque
Saturday, May 10, 2008
A torque wrench is a key part of your tool kit. Knowing how to use it for the maximum efficiency and safety can greatly enhance your mechanical performance
by RALPH WINFIELD
My first two back to basics articles brought so much response that I decided to add a third - this time on torque.
By now, most of you should have at least one torque wrench in your shop. There are many occasions when bolts or nuts must be tightened to a predetermined uniform torque.
If you change wheels or rotate the tires on your car or pickup, the lug nuts should be tightened to the correct torque, preferably in a specific torque pattern. The primary reason for this is to ensure that the nuts will not come loose. The second reason is to eliminate warping of the brake rotors, and third so the threads aren't destroyed by over torquing. (Brake rotors are those disc things that you see whenever you remove a wheel from almost all newer cars.)
If you torque a five-studded wheel, the nuts should be tightened in a crisscross "star" pattern. That is, 1, 3, 5, 2, 4 or 1, 4, 2, 5, 3.
For a six-studded wheel, the nuts should be tightened as 1, 4, 2, 5, 3, 6 or 1, 4, 6, 3, 5, 2.
Torque wrenches are great tools. They can be set to precise torque settings. Most will provide an audible click or flash a light when the preset torque is reached.
However, for some routine applications such as mounting vehicle wheels, colour-coded, torque-limiting, half-inch drive extensions are readily available.
If you haven't seen them, pay attention the next time you visit your local tire shop. They are used on impact wrenches for routine tire mounting, which is what I do too. They are great for that type of application where absolute precision is not essential. Do still use the star-torquing pattern as illustrated.
Engine work. If you do serious work on engines, I am sure you know by now the value of the torque wrench. It is imperative that head bolts be torqued in a specified stepped pattern. When properly torqued, we expect an engine head to stay in place for thousands of working hours. Gone are the days when we removed the engine head every 10,000 miles to grind the valves. Head gasket and engine valve technologies have increased significantly in recent decades.
Yes, if you remove the pistons and/or the crankshaft, you will need the torque wrench again. Main bearing and connecting rod bearing bolts must also be precisely torqued. As an example, the service manual for a Cummins B series engine gives specific three-step torque values for the head bolts and both the main and connecting rod bearing bolts.
Other simple torque needs. If you do tasks as simple as changing pivoting, flail-type blades on hammer mills or the stalk chopper on your combine, torquing the pivot bolts is critical.
I speak from experience.
Pivoting blades commonly have washers/bushings slightly thicker than the blades. It is absolutely critical that the bolt/nut be tight enough to lock that bushing. If the bushing is not locked, it will pivot on the bolt rather than having the blade pivot on the bushing. The bolt will fail!
A thrown blade can do secondary impact damage to the machine. But, more importantly, its departure will put the rotating device (chopper) out of balance. If not stopped immediately, the resulting equipment damage can be significant.
I trust that some of you have also lost a straw chopper hammer. There is only one way to know that it is gone. You have to recognize it with your butt on the combine seat!
Have you also noted that most of those short bolts that hold hammers are fine threaded? Fine-threaded bolts will hold torque better because they have a shallower thread ramp angle. Some of you will remember when plowshare bolts used to be fine-threaded to prevent the shares from coming loose.
Those large-diameter bolt rings.
All higher horsepower tractors must transfer very high torques to each one of the drive wheels. Let us assume that you need to transmit 60 hp to one 38-inch rimmed tire. A quick check of my neighbour's 130-hp tractor shows that it has a bolt ring of 16 three-quarter inch diameter bolts, each torqued to push a steel wedge out against the rim to transmit the engine torque from the axle to the rim.
The torque arm, the distance from the centre of the axle to the ring of bolts is 16 inches or 1.33 feet. The other measurement I made was the rolling radius of the big radial tire. It is 31 inches or 2.58 feet. The rolling radius is the distance from the centre of the axle to the ground.
If we assume that the tractor is travelling at five mph, the distance travelled per minute would be: D = 5 x 5280/60 = 440 feet per minute (fpm). (A handy rule of thumb is that one mph = 88 fpm.)
Since the rolling radius "r" of the tractor wheel is 2.58 feet, the tractor axle will turn "x" rpm when the tractor is going five mph.
x = Distance (fpm)/feet per rev. = Distance (fpm)/2Ï€r
x = 440/2Ï€ x 2.58 = 27.2 rpm
Going back to the simplified formula for torque in Part II of my series (Better Farming, December 2007), we find that the torque created by the engine at the axle would be:
T = hp x 5225/N = lb-ft
= 60 x 5225/27.2 = 11,525 lb-ft
If we go back to the bolt ring, the torque force that must be offset by those steel wedges to prevent the cast wheel hub from rotating within the rim would be:
F = torque/moment arm = lb
= 11,525/1.33 = 8,646 lb
Can you now see why it is so critical that those 16 3/4-inch-diameter bolts all be torqued equally and sequentially to provide the necessary grip on that rim? Please remember that for every force or action, there is an equal an opposite force or reaction. It's one of those rules of physics that's always true!
Other tractors, usually those with steel-dish wheels rather than cast ones, have at least eight large bolts in shear at the wheel-to-rim interface. As you know, the manufacturers always put bolt rings at that location to allow us to adjust wheel treads in small increments for various field tasks.
My point is that, if you are adjusting wheel treads or if the rim has been removed to replace or repair a tire, make sure that the wheel bolts are properly torqued and then rechecked after one, or at most two, days of field operation.
With the ring of bolts, if one comes loose, the rim will distort enough to reduce significantly the torque of the two adjacent bolts and the ones directly opposite. The torque reduction process is like falling dominoes - in slow motion.
Another example. Most of you do not own or operate large track-laying backhoes, but you might still be interested in this bolt torquing issue.
If you look under the cab of these units, you will see a large-diameter swing bearing with an internal gear ring. One or two pinion gears drive that gear ring to swing the complete backhoe unit and boom. Each half of the swing (slewing) bearing will have at least 30 bolts attaching it.
Based on the large torque value we calculated earlier, can you appreciate the torque loads required to turn that backhoe on its under carriage?
Now for my additional point using this example. On most of these large units, you will see that those 30 bolt heads extend out beyond that large diameter slewing bearing. Why, you ask, would anybody use two- to four-inch lengths of heavy walled pipe (spacers) under the bolt heads?
Well, there is a good reason. Those 30 bolts are torqued to very high values - in the orderof 500 lb-ft - in a very specific sequence. The bolts must literally be stretched during the torquing process. In order to get those high torque settings, the bolts must be long enough to permit sufficient stretching. Do remember that it is the stretching of the bolt that permits it to maintain or hold its torqued value.
And, no, you do not and should not use lock washers on torqued bolts. They would break and come out often before the desired torque values are reached. Lock nuts are used in some applications, such as on the 3/8-inch-diameter bolts that hold my torqued straw chopper hammers in place. The torque value is only 40 lb-ft (54 Nm).
Please remember to set the torque value on your torque wrench back to zero after use and before you put it away. This practice will assist in maintaining its accuracy for future use.
Torque multipliers are available to obtain high torque values. They use a gear set to obtain a mechanical advantage, when required torque values get into that 500 lb-ft (680 Nm) range.
Until the next lecture, continue to observe the many fascinating items we deal with in our everyday lives.
If you enjoy this mini-lecture series, please tell the editor. He likes to hear from you. BF
Agricultural engineer Ralph Winfield farms at Belmont in Elgin County.