WW1 Thornycroft restoration

This now completes the work for the “Axminster Division” on the Drag Link and the whole Drag Link assembly will now go on to the “Leicester Division” for final machining and brazing there to complete the job.

The new casting is shown alongside the pattern.

What material?

I would be a bit nervous of using cast iron in that application, though ductile iron or ADI would be fine.

(I did some fatigue testing for JCB on ADI in a previous life, and it's amazingly strong and tough)

What material?

 

I would be a bit nervous of using cast iron in that application, though ductile iron or ADI would be fine.

(I did some fatigue testing for JCB on ADI in a previous life, and it's amazingly strong and tough)

It's SG and I feel quite sure that it will be suitable!

Tony

It's SG and I feel quite sure that it will be suitable!

Just checking. Failure of that component could be quite exciting :-)

Just a little time to spare amidst other obligations – but enough time to make up the Brake Pull Rod for the Transmission Brake. We never had one of these so the drawing of it is made up from information gleaned from other “Thornys” and from photographs and measurements taken previously. These two pictures are of the Carlton Colville “Thorny”.

Very simple construction and components – a length of 1/2” diameter steel bar for the shaft – a piece of steel Bar turned down to 1 1/8” diameter for the 1” length “boss” and a standard old fashion 1/2” Whit wing nut with “Mickey Mouse” ears. The 1/2” Whit thread was screw cut in the lathe – a short threaded section of 2 BA was left on the end of the 1/2” shaft to screw into the “boss” for location purposes for welding.

Finished item and primed – a little filler around the weld to tidy it up.

Another of the original assemblies that we never had was the Foot Brake which is a Transmission Brake assembly and we have to make this from scratch. A pattern was made for the Brake Shoes using measurements and pictures gleaned from complete Thornys and from the picture in the Parts Book. The new shoes were cast at our local Foundry and are in SG Cast Iron. The picture of the complete one shown here is of the Thorny at the Carlton Colville Transport Museum.

To obtain the correct position for the Fulcrum Pin hole in the bottom of the new casting, a piece of mild steel bar, 1” in diameter (same as the Fulcrum Pin) was machined with a point on the end and inserted into the main Gearbox casting where the Fulcrum Pin will be located. Later on, the machined Brake shoe will be clamped to the Brake Drum and rotated against the point to leave an arc on the casting, locating the point where the Brake Shoe must be bored out to take the Fulcrum Pin. Although the Brake Linings are on order, they have not yet been received – they are 3/8” thick and pieces of 3/8” Dowel have been temporarily super-glued to the Brake Shoe so that we can obtain the correct distance of the shoe from the Brake Drum when it is rotated for the marking out.

The curved surfaces of the shoes to accommodate the linings were cleaned up with files and flap wheels and no further machining of those surfaces is deemed necessary.

The castings were then cleaned up in the Mill and also machined to establish datum points. All of the machining could be accomplished without moving the castings so that all of the machine surfaces will be true with one another.

The clevis’ were also cleaned out and machined to final size to take the Tension Rod and activating lever. The holes for the Pins to be drilled later.

The completed machining at this stage – and also a trial fit of the Tension Rod.

The Brake Shoe clamped up to the Brake Drum and rotated against the point of the “dummy” Fulcrum Pin.

The first Brake Show bored out for its Fulcrum Pin and trial-fitted. It rather looks as if the Fulcrum Pin will have to have its shoulder machined back a little as the Brake Shoe is over lapping the Brake Drum on the outside – nearest to the camera. Easily remedied – but not yet –wait until the other one has been done!

A couple more pictures of the machining set-up and the machined item at this stage.

The machined Brake Shoes in position. Painting next and then fit the linings.

I like the dowel idea. Quick question about tolerance. How critical is the position of the fulcrum pin hole? Alternatively can any small error be taken up by brake-pad wear without compromising braking? Robert

May i just say that I find this all absolutely mind boggling. I restore a normal car in the normal way and figure i have done something remarkable but this takes both skill and madness to a completely different level. Keep the reports coming - i am in awe.

Sent from my iPad using Tapatalk

Tim,

To correct any slight error in the fulcrum, you could make up a new fulcrum pin with the pivot part slightly eccentric, a bit like jeep brakes shoe pivots. Then the shoes could be adjusted for full contact.

Robert and Richard have both raised an interesting point - and I am not quite sure how this is going to work out in practice! You may recall that the Gearbox was recovered from a mountain in North Wales where it had been adapted to drive some kind of Water Pump. It appears that a flat drive belt to the Pump was taken off the Brake Drum and for this purpose, an extra piece was beautifully welded on to the Drum, just to make it wider. This had to be cut off again to return it to original and this was shown in Postings Nos 1376 to 1379 dated 29th July 2014.

It also appears that once the additional piece was welded on to the Drum, the whole thing was skimmed in a lathe and a bevel machined across the whole contact surface - presumably to accommodate the flat belt. We have not re-machined that surface as that would further thin the contact surface. Just too much metal would have to be taken off to get it "flat" again.

It always seemed to us that the Brake Linings when fitted would just bed in on what would be a slightly sloping contact surface. I think of the surface of any Brake Drum as quite smooth but you will see from previous pictures that the surface shows signs of the skimming so that it is not completely smooth - and whether this will cause early wear of the linings remain to be seen.

With this lorry - and also with the Dennis, the main Brake is the hand brake which works on the rear wheels and the foot brake on the Transmission is taken more as an emergency brake and is not used in the first instance. So it could be quite some time before any significant wear is apparent on the Transmission Brake linings.

We explained in the note of yesterday how the position for the fulcrum pin was worked out - but that was not at all straight forward in practice for the one on the right. Looking at the Gearbox from the Brake Drum side, then that was all straight forward for the left side, but it was not possible to swing the Brake Shoe on the right side as the Gear Box is sloping to the right on the table where it has been resting as there was insufficient clearance to swing it on that side - so the "Axminster Division" took a chance and swung both the Brake Shoes on the same left side.

So in practice, when the completed Brake Shoe was trial fitted to the right one, there is a small gap between the dowel and the Drum contact surface at the point of the Fulcrum Pin. Whether that is due to the inaptitude of the "marker-out" or if in practice the two holes in the Gearbox casting are not equidistant from the Brake Drum, doesn't matter now! The "gap" can be taken up by using a slightly thicker brake lining but a decision on that can remain for the moment until other team members have cast their eyes over it!

Tony

The "gap" can be taken up by using a slightly thicker brake lining but a decision on that can remain for the moment until other team members have cast their eyes over it!

The fire-engine had had brake linings retrofitted to the shoes, and that meant that the working diameter was wrong.

I changed the radius of the metal shoes to partially correct this (using my dad's horizontal boring machine), then bored out the relined linings to the correct radius to complete the job.

In principle you could remove the drum from the gearbox, set the gearbox up on a boring machine, clock true to the output shaft then bore the shoes to the exact diameter in-situ.

An alternative, though not so accurate, would be to wrap a spiral of emery tape round the drum and apply the brake. The problem is the lack of ideal inelastic zero-thickness emery tape.

I suspect that the linings will wear in pretty quickly. When we first took the Dennis to Brighton, I had to get underneath only about five miles out to tighten up the adjuster as I was already running out of travel! I am sure that the drum will polish up and the top edge of the lining will quickly disappear until it is all in contact.

Time will tell!

Steve

With the Transmission Brake Shoes now completed and adjusted to hang in their final positions, the Keeper and Stiffening Plate can also be trial-fitted. This item is not listed or described in the Parts Book but our earlier research from other Thornys showed us what was required. The first picture here shows just half of the equivalent Plate on the Carlton Colville Lorry.

This plate drops over the Fulcrum Pins, outside the Brake Shoes and serves two purposes – it retains the shoes in their correct position and also helps support the Pins when pressure is applied to the brake.

It is made from a length of 3/8” x 2” Steel bar but with four discs of 1/16” thick, 2” diameter steel, silver soldered on the bar – one on each side of each end. The discs were held in position with temporary nuts and bolts for the silver soldering after which they were removed.

The original part is similarly thicker at the ends and was presumably made by forging – our silver soldered discs at the end of our new part is designed to replicate the original.

With the silver soldering completed and the temporary nuts and bolts removed, the holes in the Keeper Plate were opened up by drilling and boring to their final diameter of 1”.

Before completing the shaping of the Plate, it was slipped over the Fulcrum Pins, just to make sure that it fitted. Then it was a case of rounding off the ends – a nice filing and hacksawing job!

A drop of paint and part completed. The Brake Shoes line up nicely with the drum – next job to paint the Shoes before the linings are fitted.