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WW1 Thornycroft restoration


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We didn't actually get to the limit of the gauge. At least, I don't think we did.

 

Barry's post has a table of loads and pressures, and peaks at 20 tons which is more like I would have expected.

However the PSI numbers on a 14" ram don't seem to match at all.

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Barry's post has a table of loads and pressures, and peaks at 20 tons which is more like I would have expected.

However the PSI numbers on a 14" ram don't seem to match at all.

 

I would have expected the diameter to be quoted also.

 

It would follow that the load required to press them off was dependant not only the width of the tyre section, but its diameter.

 

A friend of mine has a Foden D type steam tractor, it still has its original press on rings which have been recovered with new vulcanised tyres some 20years ago. The downside is that the rubber is like chewing gum and cuts up something rotten. He wants to do it again, but doesn't want to send the wheel to be done as it has to go in the autoclave and will bugger the paint.

 

I've suggested turning the rubber off in a manly lathe, then pressing off the rings and sending them only for vulcanising. These wheels are over 4 foot diameter, and may be on the large side for Barrys press.

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It would follow that the load required to press them off was dependant not only the width of the tyre section, but its diameter..

 

You would expect that, but it might not actually be true.

 

The limiting factor is the hoop-stress in the hoop. Assuming that the hoop is at its yield point, then the hoop stress depends on the hoop cross-section, ie width and thickness.

 

The frictional force depends on the radial component of the hoop stress. Classically friction = (mu) x R. Notably the surface area does not feature in the equation.

 

So, a wider hoop may be under more tension, and will grip the wheel more tightly, but there will not be any more friction.

 

However, it gets even more counter-intuitive than that. As the diameter increases the radial component of the circumfrential stress reduces, so a larger ring of the same thickness and width will actually grip less tightly than a smaller one.

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You would expect that, but it might not actually be true.

 

The limiting factor is the hoop-stress in the hoop. Assuming that the hoop is at its yield point, then the hoop stress depends on the hoop cross-section, ie width and thickness.

 

The frictional force depends on the radial component of the hoop stress. Classically friction = (mu) x R. Notably the surface area does not feature in the equation.

 

So, a wider hoop may be under more tension, and will grip the wheel more tightly, but there will not be any more friction.

 

However, it gets even more counter-intuitive than that. As the diameter increases the radial component of the circumfrential stress reduces, so a larger ring of the same thickness and width will actually grip less tightly than a smaller one.

 

I would hope that the hoop stress is well below the yield point! (Otherwise you're into the permanent deformation region if the stress increases any further - which it will during use I'm sure.) But yes, the hoop stress is limited by the strength of the material (which increases with the cross sectional area (i.e. width of tyre x thickness of band)) and the gripping force on the wheel is determined by the radial force and the area of contact. I'm sure that the larger diameter tyre will grip as well as a smaller one because of the increased length of the circumference and hence a larger area in contact (which would compensate for the reduced radial stress) - otherwise they'd make the hoop thicker to compensate and avoid the tyre spinning on the wheel or slipping off.

 

(It's nearly 40 years since I had to know this sort of stuff, so don't ask me for the formulae or I will have to resort to Google!):-)

 

Chris.

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I would hope that the hoop stress is well below the yield point!

 

I wouldn't bet on it. There is a lot to be said for letting things size themselves, and solid tyres seem like a case where allowing yield in the relatively weak band would be a good way to cater for wheel size tolerances while ensuring the maximum grip.

 

For most steels the tensile strength increases after yield, so until the reduction in area is significant the load carrying capacity of the component increases, and the stress distribution is equalised.

 

But maybe a real mechanical engineer should step in, I just play one on the Internet :-)

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Well I am an electrical engineer so I can talk nonsense on this subject if I want to! I did think Andy's statement that the hoop was at yield point was a little far fetched purely from a practical point of view. In most cases the hoop is just a rolled steel band welded together. For a 720 wheel, the band pre-rolling, has to be 2.262 metres long. As a rule of thumb it is said that the you reduce the length of the band by 1mm so that when it is welded up and pressed onto the wheel, it will be a 'good tight fit'. I would have thought that the ability to achieve sub millimetre precision for this kind of work would have been very difficult to so that the hoop strain was very variable or 'all over the place' for the want of a better description.

 

(But is this just an argument in Andy's favour saying that you always size your band to be at the yield point or beyond so precision isn't required?).

 

A question for those who know better. Were the bands ever bored on a big lathe after rolling to achieve the tolerances needed? All the bands that I have seen have not been machined.

 

 

....A quick calculation shows that the tyre band only has to be 24 thou (0.6mm) smaller on the diameter for the yeild point of the band to be reached. Not much margin of error to play with.

 

 

Barry.

Edited by Asciidv
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The comment on the band or hoop for the tyre being a rolled piece of steel then welded, doesn't fit the description of the wheels we have. These are made of cast. As such the skill in casting these to fit with fine tolerances to be then pressed on to the wheel shows through.

The Leyland rear rims have a series of raised ridges on outer face to assist the bonding of the rubber to the metal. Thornycroft bands have a raised ridge about the edge of the band to stop the tyre sliding off should it become loose, or to protect the edge of the band and tyre from damage These points show the bands as being cast and not a rolled item

Doug

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The comment on the band or hoop for the tyre being a rolled piece of steel then welded, doesn't fit the description of the wheels we have. These are made of cast. As such the skill in casting these to fit with fine tolerances to be then pressed on to the wheel shows through.

The Leyland rear rims have a series of raised ridges on outer face to assist the bonding of the rubber to the metal. Thornycroft bands have a raised ridge about the edge of the band to stop the tyre sliding off should it become loose, or to protect the edge of the band and tyre from damage These points show the bands as being cast and not a rolled item

Doug

 

They are steel, probably came off the roll at the steelworks with the ribs on then rolled and welded into hoops before the rubber was added.

 

You can also tell they are steel when you cut them, and by the way they corrode and weld.

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The comment on the band or hoop for the tyre being a rolled piece of steel then welded, doesn't fit the description of the wheels we have. These are made of cast. As such the skill in casting these to fit with fine tolerances to be then pressed on to the wheel shows through.

The Leyland rear rims have a series of raised ridges on outer face to assist the bonding of the rubber to the metal. Thornycroft bands have a raised ridge about the edge of the band to stop the tyre sliding off should it become loose, or to protect the edge of the band and tyre from damage These points show the bands as being cast and not a rolled item

Doug

 

That differs from what we have Doug. Can you post some pictures please?

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They are steel, probably came off the roll at the steelworks with the ribs on then rolled and welded into hoops before the rubber was added.

 

You can also tell they are steel when you cut them, and by the way they corrode and weld.

 

My comment regards casting was towards the method, not the materal used. I agree with you on the point of rusting,and the use of steel. However can see no sign of a welded join indicating a flat plate as the origin. Your mention of rolling the ridges is a valid point and if done hot could also account of the raised edges. Could the hoops be also be joined in the hot state, similar to a blacksmith forging joint, but done mechanically.

If a factory was producing hundreds of hoop rims at a time some form of mechanisation would be expected. An updated version of the blacsmith making cart wheel hoops (tyres) individually and by hand.

I will wander off latter today and photograph the different tyres.

Doug

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Just for a change, a little bit of brasswork.

 

A while back, we were very kindly given a super fuel filter, not quite the right one but one which would do the job with no work and look presentable. However a couple of weeks ago, Tim found this on Ebay:

 

DSCN4647c.jpg

 

Comparing it with the parts book, it is exactly the right one although a bit ropey so we decided that it must be brought back and fitted.

 

Thorny Fuel Filterc.jpg

 

Sorry about the awful pictures. I wasn't having a good day. First step was simply to strip it down to find it full of muck and corrosion, as expected. I pickled the gauze in sulphuric acid to dissolve off the corrosion and then straightened it gently using a small screwdriver and my fingers. Someone had unscrewed the filter previously with a pair of pliers and had crushed the bowl. They had also drilled out one end and tapped it for a tap.

 

DSCN4649c.JPG

 

I started with the bowl and, after annealing it, held it over a bit of bar in the vice and then tapped it with a hammer and chunk of steel to push the dents out.

 

DSCN4656c.JPG

 

Not perfect but an improvement.

 

DSCN4657c.JPG

 

Then I turned up a threaded spigot with the correct thread on the end and screwed it in with Loctite. I could have silver soldered it but the suction pipe is only soft soldered into the cap and I didn't want to disturb it.

 

DSCN4659c.JPG

 

DSCN4660c.JPG

 

Finally, I turned up some new union nuts and nipples.

 

DSCN4663c.JPG

 

DSCN4722c.JPG

 

A quick polish and it has gone into store ready for when we need it.

 

DSCN4724c.JPG

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Different wheel types.

It would appear that the difference we are seeing in hoops could be from different tyre manufacturers.

The Leyland hoops are ridged , although in this example very worn. These are flat with no edge raised section. In comparision the 1912-13 Thornycroft has well defined edge sections very noticeable in the photo without the tyre. Note also the bolted holding lugs of the time. The same pattern of the raised edge is on other Thornycroft front wheels but not so on the rear of the 1920's J chassis.

Wheels steel band Leyland alt eml Nov  2014 028.jpg

Wheels steel band Thornycroft alt eml Nov  2014 025.jpg

Wheels steel bands thornycroft 12  rear alt eml Nov  2014 033.jpg

Wheels steel bands Thorycroft 12 alt eml  Nov  2014 032.jpg

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I have been following the debate about how the metal ring that the solid rubber tires are moulded onto is made and no one has yet suggested that they could have been made without a join in the same way that railway wheel tires were (and are) made.

 

You start with a lump of iron/steel of exactly the right weight. Heat it to red hot and squash it into a disk. Poke a hole in the middle with a pointy thing and then enlarge the hole with bigger pointy things (without actually removing any metal - think how you might do it with a lump of clay) untill you can get the hole over one of a pair of specialy designed rollers. You then keep rolling it untill it is the circumference and width that you want. It is very easy to put whatever profile you want on the outside with a suitable outer roll, and a skilled operator can make the circumference whatever is needed with very simple gauges. There is no join and the 'grain' of the metal is circumfrencial which is ideal.

 

Remember that 100 years ago forging and rolling were a much better way to make something the shape you wanted because energy was relatively cheap (to make things red hot and then squash them) but welding, machining and cutting were much cruder than we are used to so avoided as far as possible. I know that chain manufacture was perfected with forge welded joins as were the tires for wooden cart wheels but the tolerence required for the steel bands for motor vehicle tires would have been much tighter. Also solid rubber truck tires might seem crude now but they were almost high tech then so deserved the best processes.

 

David

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Thank you David for the description of railway tyre making,. As you say it would be a logical step to use the same process for vehicle requirements.

In all the old books and publications I have this is one area that is missing. No photos or line drawings of the machinery as used. I had assummed it was a casting process as no joint is visable, or some system in rolling and mechanicaly "black smithing " the join when hot.

Perhaps we need to re engineer the process for our vehicle needs. It would make a great display item at a show event. Selling them as souvineers though many suffer from a lack in demand!

Doug

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I have two words to say to you; Barrel Hoops :-\

 

The same basic forge and roll process that the blacksmith used to make barrel hoops from steel bar would also have worked for the steel tyres for wooden rims, and then to make the steel base for your solid tyres.

 

I'm reasonably sure the basis for the ring will be steel of some kind, as cast iron, while very strong in compression, is lousy in tension, and this extends to many of the more exotic iron grades too.

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As Doug has pointed out the tyre band is not a basic hoop. This extract from the Dunlop solid tyre catalogue illustrates the variations.

 

tyre profiles.JPG

 

The American style tyre has a flat band but with raised edges to keep the tyre in place. The Dunlop style tyre has dove-tail grooves to keep the rubber in place.

 

How do you roll dove-tails into a band?

 

Barry.

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How do you roll dove-tails into a band?

 

Barry.

 

With special dove-tailed rollers of course! :???

 

Seriously though, do you think it might be a two-part process where the grooves are rolled first and then the ridges given a squeeze which spreads them out at the top?

 

Steve

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In posting no. 1428 of the 12th September, we showed the constituents of the swinging arms of the front axle. Each King Pin is contained within two bushes and the top one in each case is unusual in that incorporates the bottom section of a Thrust Bearing. These are badly corroded and beyond redemption and this sort of fitting does not seem to be available on the commercial market. In fact, no one seems to have ever heard of them!

 

Three of the original four bushes were of a hard steel and the other one was of bronze. We decided to make four new ones using bronze for them all and that the top bushes would have to be made to incorporate a standard readily available lower part of a Thrust Bearing. The King Pins have been skimmed since they were removed so the new bushes must be of a slightly reduced internal diameter to suit the now very slightly smaller King Pins.

 

A slight shoulder has been left on top of those bushes on which the bottom part of the Thrust Bearing will be a simple push fit. We have not yet pushed the Bearing on - but it will go! The sketch should show the general layout.

img004_zpsd4755beb.jpg

DSCN1181_zpsa91a4c99.jpg

DSCN1178_zps23747e53.jpg

DSCN1179_zpsba7390f1.jpg

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