Supporting Headstock Ironwork Assessment
It may be rusty and rotten looking, all trace of threads have rusted away and above the nuts the unused bolt section has rusted away to a pencil stub. Is it strong enough? A bell can exert four times its weight on its suspension. If it is wrought iron should have a minimum yield stress of 36,000 lbs/sq ins but probably better. Mild steel is 56,000 minimum and good wrought iron is similar.
Take a really bad case, six fixings rusted down to 0.25" under the rust at the thinnest point, that's 0.049 sq inches left. So for 6 fixings, the area is 0.29 sq inches. Multiplied by 36,000 and divided by 112 that gives 94 cwt. Surprisingly we do not have a tensile strength problem on the basis of a 10 cwt bell and a 2:1 safety margin which comes out at 80cwt. And that being rather pessimistic about the Iron quality.
How about the cross bars under the headstock? Typically corrosion results from condensation in towers and it is easy to see the more exposed iron. The more exposure the more the corrosion, so the argent bolts buried in the stock and the cross bars under it suffer least. Only the visible exposed ends are at risk and can be visually checked.
How about the threads? It is almost always possible to remove iron nuts, which do not seize up like modern steels. It is sometimes necessary to hold the top of the bolt with a Stilson (but NOT if there is any thread to be damaged), and or smack the nut against a block to brake the rust line. It will usually be found that the threads inside the nut are virtually free of rust. A Whitworth thread will retain its strength with 5 turns of undamaged thread. The proportion of strength left will be the initial strength x the number of full turns of good thread divided by 5 up to 5 maximum. For example, the bolt was originally 1/2wit, although 9/16 would be more normal. Core diameter 0.45 cross section, about 0.16 inch at 36,000 yield gives 51cwt/ bolt. Thread pull out is typically 80% of that so allowing 50% is reasonably conservative, say 25cwt but if only 3 full turns are left that takes us back to 12.5cwt but there are 6 fixings giving 75cwt. With a 2:1 margin that is OK for a bell up to 9cwt.
OK in theory but can it be tested? After all, I am going to stand under this thing and my life may depend on it not letting go! Yes fairly easily but there is something more to understand here. If a nut is clean and oiled the clamping force it achieves is related to the applied torque. Clamping torques are much less than yield strength because of friction and twisting torque applied greatly reducing the force that is generated. This being the case it is not possible to prove the strength margins to their full extents. It is, however, possible to fully assess cannon cross bars and headstock softness due to worm and decay. The test is to take off the headstock nuts one at a time and clean and oil the threads to make sure all are running freely, and then torque up to a value sufficient to provide enough clamping force to guarantee that X4 bell weight is being supported with some safety margin. If some softness appears before the required torque is achieved, that is movement of the torque wrench fails to generate the expected increase of pressure, the limit has been found and plastic deformation is occurring. No more force can be applied or a breakage will result.
Because of all the variables involved the figures below are subject to considerable error. What is to be noted is that torque and clamping force are nominally linearly related until Hook's Law is exceeded and plastic deformation starts. So applying and subsequently removing the extra torque can be used to prove more margin. Similarly do not use the torque figures below for badly corroded bolts if a lower torque will be sufficient to prove a decent safety margin for the weight of bell and number of fixings used. Comments refer to 6 fixings; figures are for a single bolt.
- 7/16 wit 14tpi 20 ft Lbs Clamping force 25cwt
- A 10 cwt bell would be proved by a total clamping force of 80cwt, a 2:1 margin, this requires only 11 ft lbs
- 1/2" wit 12tpi 30 Ft Lbs Clamping force 33cwt
- A 10 cwt bell would be proved by a total clamping force of 80cwt, a 2:1 margin, this requires only 12 ft lbs
- 9/16 wit 12tpi 45 ft Lbs clamping force 42cwt
- A 10 cwt bell would be proved by a total clamping force of 80cwt, a 2:1 margin, this requires only 14 ft lbs
- 5/8 wit 11tpi 65 ft Ibs clamping force 56cwt
- A 10 cwt bell would be proved by a total clamping force of 80cwt, a 2:1 margin, this requires only 16 ft lbs
- 3/4wit 10tpi 100 ft Ibs clamping force 73cwt
- A 20 cwt bell would be proved by a total clamping force of 160cwt, a 2:1 margin, this requires only 36 ft lbs.
These minimums are tiny torque figures, much lower than would normally be applied to bell bolts, and demonstrates the huge margin for safety there is in typical rusty wrought iron bell supporting work. Torque wrenches for square nuts are a bit difficult but these low torque levels can be achieved with a spring balance and an adjustable 18" adjustable wrench. The 18" wrench and my Sampson 20Kg fisherman's spring balance gives up to 55 ft lbs, because bolt centre to hole in handle is 15".
Other Thread Formats
Some very old work will have blacksmith made threaded parts with less turns per inch. This will not be as strong as Whitworth but the loss will not be huge. As an example if a 9/16 bolt were only 8tpi instead of the wit 11tpi the clamping force resulting from a given torque could be reduced by 8/11 (0.72) and the maximum possible torque is reduced slightly because a deeper thread would reduce the core diameter and hence the resistance to plastic deformation. There will also be a reduction in clamping force because the angles used within earlier thread forms. This had more friction and therefore a worse torque to clamp ratio.
I would suggest reducing the clamping force by 50% would be about as bad as it will get. Torque will still prove a decent safety margin, although the 5/8 X 8 tpi bolt may need up to 32 ft Ibs to do it. The arguments regarding thread pull out are less affected by thread form, and the number of thread turns required for a given proportion of the ultimate pull out strength will be reduced by a coarser thread as the engaged thread length is the controlling factor. Metric bolts take higher torque because of better steel, and should generate about 20% more clamping force for a given torque.
Obviously if adequate clamping force can be obtained then despite all contrary appearance the headstock still retains enough hardness to be strong enough, and will be as long as there are NO significant cracks between canon cut out and the area above the gudgeon, and no vertical cracks. Horizontal cracks and shakes are not normally significant as the bell retaining ironwork also clamps the headstock together. Typically a failing headstock will allow the bell co come lose. When tightened again it will come loose again, probably not immediately it is rung, the torque test proves that will not happen, but after some months, because of progressive crushing within the much eaten and decayed timber. Loss of bolt tension in this way condemns the headstock, so if it is a bit under strength do not over load it with more than necessary torque to clamp the bell harder than 6 times its weight.
Steel headstocks can cause problems with bell bolts if excess rusting occurs. The resultant rust expansion can overload the bolt and break it, or make a sudden failure likely.
Oh and a final point, sudden catastrophic failure of several fittings occurring simultaneously is unheard of. I have found failed ironwork when doing routine maintenance. It is very unusual and manifests by something that when checked for tightness has virtually none and falls apart at an attempt to tighten. The failure, is safely and easily found and even today getting old ironwork repaired is not a big problem. From the figures above it can be seen that the bell will normally be safe even without one pair of its fixings.
A Little Perspective on Safety Margins
Industrial lifting equipment, hoists cranes lifting straps and the like are safety critical and unlike bell fixings have a single point fail possibility a bell has a minimum of 4 fixings to its stock. These devices must have a safety margin as people working around heavy lifting equipment would be in mortal danger if a fail occurred. They only need to be proof tested at 1.25:1 just 25% over full load and can then legally be used. A normal design margin is 2 but can be reduced can be reduced and is not mandatory.
I welcome any suggestions.
Roderic K Bickerton, [email protected]
This information has been reviewed by a professional stress engineer and is conservative. Where material properties cannot be accurately assessed, I cannot take responsibility for its accuracy. It is not to be used in any way other than for discussion.
- Experiments on the Strength of wrought iron and of chain-cables, Commander L A Beardslee USN. Pub New York 1879, available on line Reese Library of the university of California.
- This gives the yield stress as 55000 lb per sq inch for a 1" bar.