Applying torque

Sam

There are some rather clever, and distinctly expensive, bolting systems with provision for measuring the stretch when the far end is buried. The simplest way being to drill the bolt almost all the way through so an internal micrometer or special gauge can be pushed down it. Some rather fascinating offerings for really big bolts that I don't understand. I guess when you are bolting a bridge or similar structure together its advisable to get things right.

Smaller things are generally arranged with both ends of the bolt accessible so direct measurement of stretch is easy. I believe Bristol were one of the earlier exponents of this on the bolted up crankpins for the master rod on their radial engines. With up to a couple of thousand hp transmitted via one crank pin its worth taking a bit of trouble.

In general bolts and similar steel rods have two yeild points. One where the extension starts to seriously diverge from the linear expectation of Hookes law followed by a second one at a little higher stress where the material finally gives up and becomes plastic. There is usually a short, flattish, section of the stretch curve between the two sections.

Stretch bolts are made from specially formulated and heat treated material to have a relatively long extension region after the initial yield with comparatively little change in the stress needed for further stretch. The bolt is still elastic in this region so will recover if you don't stretch it far enough to go into the third part of the curve up to the final plastic yield to breaking.

It won't recover past the initial yield which is why you can't, in practice re-use them. There are theoretically arcane methods of doing so but you'd be mad to try.

The extra angular turn needed when fitting stretch bolts is to ensure that the bolt is safely in the stretch zone and able to respond to changes without running out of elasticity.

For example the old all alloy push rod V8 engine in my Range Rover has two lengths of stretch bolts to hold the cylinder head on. The manual calls out 50° turn to apply the correct stretch, near enough 10 thou / 0.25 mm. The differential expansion rates of the steel bolts and alloy engine stretch the short bolts by about 5 thou / 0.12 mm and long ones by about 9.5 thou / 0.23 mm between cold and running temperature. Objectively the Rover engine runs the stretch bolts fairly close to the limits. It doesn't take much overheating to drive the bolts out of their recovery range.

Hence the reputation for head problems after overheating when serviced by folk who shouldn't be let loose on anything beyond a Morris Minor.

The oft touted use of non stretch studs (ARP et al) has its own issue. The differential expansion pretty much doubles the clamping forces on the gasket and pushes the stud very close to their yield stress. I'm less than convinced that accepting the very wide variation in clamping force on the head gasket is a better thing than the extra care needed with stretch bolts. Pushing hot running temperature up by 10°C and forcing fast warm up to meet emission regulations was not, in engineering terms, a good thing for the motor.

Clive

Edited By Clive Foster on 26/11/2022 16:54:32

In my teenage years, like most youth of the 60’s, I was obsessed with motorcycles, I remember one particular vehicle that I owned was a 500 cc single cylinder Matchless. The big end failed and I had to strip the engine to replace it, when subsequently reassembling I think I was fitting the cylinder head and tightening the bolts with a ring spanner and suddenly one bolt sheared whilst I was tightening it, the top of the bolt took the spanner out of my hand and they both hit the garage ceiling as there was some considerable pent up force locked into the bolt when it let go. Needless to say that I didn’t possess or have access to the correct requisite torque wrench which might have saved the bolt from letting go in the manner that it did. Later in life I spent many years working on aircraft of all shapes and sizes, torque wrenches that were correctly calibrated became the norm, particularly when fitting the large main wheels on transport aircraft where the wheel bearing preload was critical to prevent embarrassing failures. Dave W

Sulzer marine engines used to have plain nuts with Tommy bar holes on the main bearings. A hydraulic jacking device was screwed onto the end of the stud, over the nut, which was pumped up to a pressure that gave the right tension. The nut was then hand-tightened, and the pressure released.

Two memories from my (too brief) time at BSA in the late 1960s:

Their twins, and Triumph equivalents, had through big-end bolts in light alloy conrods, that were measured for length and tightened to a specified increase in length. The nuts were all-metal stiff nuts, (which vary a lot in initial stiffness) so this ensured that the applied stress was independent of nut-stiffness.

The big-end nuts (on the ends of the taper-ended crankpin) on the BSA Gold-star singles were tightened to 180 foot-pounds. This required a special crank-holding jig, bolted to a bench, which bolted to the wall - and a 4 foot tube over the end of the spanner.

Happy days

Cheers, Tim

Hi, the biggest torque wrench I've used had a 1" drive and was about 6 ft long, the handle was about a third of the length and was hinged onto the business part, which would move once the setting was reached. It probably weighed about 3 Kg. I can't remember how high it would go, but probably 1000 or so NM. It was mostly used for torquing M24 12.9 high tensile hex head bolts with high tensile Nyloc nuts and hardened washers under both the head and nut, there were about 30 or so of these bolts holding two parts of a big rotary kiln together, but every so often, one or two would snap and had to be replaced as soon as! We had to put up a tower scaffold to replace these bolts and it wasn't much fun tightening them up at 650 NM. as one of us had to use the torque wrench and the other guy holding the other spanner to stop the bolt turning. Not something you really wanted to be dragged out of bed for at about 2 O'clock in the morning.

Regards Nick.

The studs holding down steam turbine top casings I worked on had a small hole drilled almost all the way down the centre for an extended dial indicator plunger to drop down into and measure the stretch while half a dozen of us pushed on a very long scaffold pole slipped over the spanner handle.

Initial tightening was done with a flogging spanner and 28-pound sledge hammer that came with its own 6'6" Albanian body-builder lad to swing it. I was the one who had to press my boot down on the flogging spanner to stop it flying away and hope his aim was as good as his weight lifting.

Edited By Hopper on 27/11/2022 02:25:56

Yield tightening is when a fastener is tighten to just beyond the elastic limit,(begins to yield ) and takes a permanent extension. It is the most efficient use of the material.

The 32 spindle Unbrako machine that we used monitored torque and rotation, so that once the rate of torque increase vs angular rotation decreased, (i.e.no longer linear, the machine stopped ) from the intensive work that we had done, on every spindle, we knew that each bolt had increased in length by about 0.002"

This meant that each fastener now had a tensile load applied of 9 tons.

If you overtighten a fastener, you feel that the rotation is taking place, but then resistance (torque ) is no longer increasing at the same rate. You have loaded it beyond the elastic limit and put it into yield.

If you persist, it will fail. (As a youngster, I failed several 1/4 BSW bolts in this way.)

Monitoring the stretch in a fastener, as long as the material characteristics are known, provides an indication of the tensile load within that fastener.

The clamp load is determined by the coefficient of friction between the two members of the fastening (Delendent on the materials and the surface finish, and the depth of then thread)

A 1/2 x 40 ,tpi thread in brass will fail at a lower torque than a 1/2 UNF (20 tpi ) in even mild steel.

Howard

Edited By Howard Lewis on 27/11/2022 12:45:36

I know it's not intuitive, but within sensible bounds tpi doesn't affect the tension torque relationship that much. The reason is that somewhere about 80% of the applied torque is absorbed in friction, and the finer the thread relative to the diameter the higher is this percentage. I doubt 1/2" 40 tpi is sensible bounds. I have a very erudite paper on this, if anyone wants a copy send me your email address via pm and I'll scan it.

Then we come to the REALLY clever thing , how does a rubber O-Ring seal pressures of 1000s of psi

with only a hand tightening of the connecting nuts ?

By operating on a small effective area (it's per square inch)

regards Martin