Complete new stub axles have up to now been available, but the supplier has stated that there will be no more produced. An option is to replace the spindles, which are now well over 70 years’ old. Eric Worpe explains how it is done.
Several TC friends have decided to replace their stub axle spindles after hearing about occasional spindle failures. I became concerned after seeing that the newly installed spindles had turned blue from overheating, despite being fitted by a respected engineering company.
The replacement spindles are intended to be an interference fit in the bored out old stub axle forgings. These have to be expanded by heating to allow the new spindles to be dropped in.
The degree of heating should be well below the original tempering temperature of the carbon-alloy steel forging. The original heat treatment would have been chosen to give both strength and fatigue resistance, qualities that could be degraded by overheating.
The new spindles on the other hand, are either left at room temperature, or preferably pre-cooled in a freezer compartment. In the case of the blued replacement spindles, the stub axle forgings must have been heated to a point where they were able to transfer enough heat to raise the temperature of the spindles to 300 deg. C.
My concerns persuaded me to have a go at fitting new spindles; these were obtained from Bob Grunau in Canada and a friend kindly volunteered his broken stub axle for the experiment.
The stub axle was secured in a machined disk held in the chuck of a lathe. Photo 1 shows the boring out operation on the stub axle. This is nerve-racking, as the bore has to be machined quite accurately to give an interference fit of between 1.5 and 2.5 thou. of an inch. Photo 2 shows the bore being measured.
Photo 1 – the boring out operation.
Photo 2 – measuring the bore.
The main purpose for the interference fit is to clamp the spindle so that it is unable to rotate when the spindle’s nut, securing the ball bearings and the spacers to the stub axle, is tightened. The new threads (3/4” UNF) are more substantial than the original threads, and will need new TD/TF stub axle nuts, which should be tightened to 125 ft.lb. to ensure rigidity of the whole assembly.
When the nut is correctly tightened, the inner races of the ball bearings and the spacers form the effective diameter of the spindle as far as bending forces are concerned. This reduces the dynamic fatigue stresses experienced by the actual spindle, a very good reason for not leaving out the spacer when taper bearings are fitted.
The new spindle is secured by its button head, which is pulled up against a machined shoulder in the stub axle when the spindle’s nut is tightened.
Heating up the stub axle and forcing the replacement spindle into position is a one chance only operation. If the spindle is slightly misaligned whilst being driven into the stub axle, then it could seize up as the stub axle shrinks due to cooling down and thus fail to seat correctly.
To help alignment, a jig was fabricated (Photo 3 below), from a heavy brass tube in which a nylon piston was free to float, but supported by a compression spring in contact with a sturdy base, which screws in to the bottom of the tube.
Photo 4 – shows the nylon piston floating at the top of the tube with the stub axle about to be located on the rim of the tube.
The nylon piston has been bored out to hold the new spindle so that when the spindle is inserted through the stub axle into the piston, its shoulder floats just free of any contact with the stub axle (Photo 5). This should set up and maintain the alignment of the spindle as it is forced into the stub axle to the point where the button head makes contact with the shoulder in the stub axle.
It’s important that the spindles’ nuts are correctly tightened, as the integrity of the stub axle now depends on this aspect. The extent of the interference-fit and how to achieve it has been worked out as follows:
If we choose to heat up the stub axle in a domestic oven, or in a hearth to 220 deg. C, we can aim for a temperature difference between the stub axle and spindle of some 200 deg. C, (allowing for a frozen spindle and some cooling down of the stub axle during the transfer to the alignment jig).
If the coefficient of expansion of steel = 12 x 10(-6)/deg. C, then on a diameter of 1.125”, 200 deg. C would produce an expansion of:
1.125 x 200 x 12 x 10(-6) = 2.7 x 10(-3) or 2.7 thou. of an inch.
So, an interference fit of 2 thou. +/- 0.5 thou. is a practical starting point.
Machining the stub axle bore to such limits calls for considerable care. The cutting tip sits on the end of a long boring bar and consequently lacks rigidity; any dulling of the cutting tip results in a deflection error. Fortunately, I obtained a David Brown adjustable floating reamer (Photo 6), which enabled an accurate final cut of the bore.
Photo 6 – the David Brown adjustable floating reamer.
After the insertion of the new spindle, a small part of the button head is often needed to be ground away to allow clearance for any “flashing” on the axle eye’s forging.
Some compensation in the location of the spindle in the stub axle is needed if taper roller bearings are to be fitted, due to their inherent offset.
I’d like to thank Clive Manser for taking photos 1 to 5 and giving me encouragement and Roger Warren for the many chats on machining that have enabled a small part of his engineering expertise to be passed on to me. He describes me as “a bit of a Fred Dibnah”, which he might think is a term of disparagement, but is, in fact, high praise to me.
Ed’s note: Eric tells me that he is prepared to do a limited number of these stub axle replacements. He can be contacted at: e.worpe(at)btinternet.com [Please substitute @ for (at)]
Eric also has a few TC kingpin bushes for sale – these are bi-metal bushes, known as ‘wrapped bushes’ and are exactly the same as fitted when the cars were new. Contact details as above.
For a set of 4 the cost is 32 GBP. For orders of between 3 sets of 4 and 9 sets of 4 the cost is 30 GBP per set of 4. For orders of 10 sets and above the bushes can be purchased for 28 GBP per set of 4.
Postage at cost on all orders.
These prices are not very much above cost and only made possible by arranging a bulk order.
Bi-metal ‘wrapped’ kingpin bushes – note the oil/grease groove which has a spur take-off that feeds lubrication to the thrust faces of the beam axle’s ‘eye’.
Bi-metal ‘wrapped’ king pin bush showing the spur groove that feeds the thrust washer.
Ed’s further note: Quite a few readers will be wondering who on earth is Fred Dibnah! This from Wikipedia….
Frederick Travis Dibnah, MBE (29 April 1938 – 6 November 2004) was an English steeplejack and television personality, with a keen interest in mechanical engineering, who described himself as a “backstreet mechanic”. When Dibnah was born, Britain relied heavily upon coal to fuel its industry. As a child, he was fascinated by the steam engines which powered the many textile mills in Bolton, but he paid particular attention to chimneys and the men who worked on them. He began his working life as a joiner, before becoming a steeplejack. From age 22, he served for two years in the Army Catering Corps of the British Army, undertaking his National Service. Once demobilised, he returned to steeplejacking but met with limited success until he was asked to repair Bolton’s parish church. The resulting publicity provided a welcome boost to his business, ensuring he was almost never out of work. In 1978, while making repairs to Bolton Town Hall, Dibnah was filmed by a regional BBC news crew. The BBC then commissioned a documentary, which followed the rough-hewn steeplejack as he worked on chimneys, interacted with his family and talked about his favourite hobby—steam. His Lanky manner and gentle, self-taught philosophical outlook proved popular with viewers and he featured in a number of television programmes. Toward the end of his life, the decline of Britain’s industry was mirrored by a decline in his steeplejacking business and Dibnah increasingly came to rely on public appearances and after-dinner speaking to support his income. In 1998, he presented a programme on Britain’s industrial history and went on to present a number of series, largely concerned with the Industrial Revolution and its mechanical and architectural legacy. He died from bladder cancer in November 2004, aged 66. He is survived by his five children from his first two marriages.
Fred Dibnah – picture courtesy of wiganworld.co.uk
So, now you know!
I have sent a small donation to Wikipedia as I have used them before and it is not fair to expect to use their research for free when they rely on donations to keep afloat.
I have also sent a small donation to Grace’s Guide. This registered charity is the leading source of historical information on industry and manufacturing in Britain. This web publication contains 146270 pages of information and 231762 images on early companies, their products and the people who designed and built them.
I used Grace’s Guide for my research in compiling the Armstrong Shock Absorbers article.