Pony Truck Appeal
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We are grateful to Michael Hughes from the BR Class 8 Steam Locomotive Trust for the reproduction of these photos.
The pony truck is the "2" of a 4-6-2 Pacific locomotive. British Railways called it a pony truck whereas it really is a Delta trailing truck. Delta because the weight of the rear of the locomotive is carried at 3 points.
- A bearing surface around the front pivot which is located below the front of the firebox.
- 2 oval bearing surfaces on the rear beam of the pony truck frame. You can see these on the photograph above.
On the oval bearing surfaces are 2 bolster cups which can slide on the oval pads. The bolster cups engage with bolster domes that are bolted to gussets at the rear of the locomotive frames. This system allows the pony truck to pivot around the forward pivot as it negotiates curves in the track.
The Class 6 Clans and the Class 7 Britannias were fitted with an all fabricated pony truck frame with leaf springs. The fabricated frame caused problems as cracks were experienced around the pivot (see Britannia by Philip Atkins Chapter 16). Also the leaf springs gave a hard ride.
For 71000, Duke of Gloucester, BR came up with a revised design with a cast radius arm (with the front pivot) bolted to a fabricated frame. In addition coil springs were used for the suspension springs instead of leaf springs. This is reported to provide a much better ride. This design was intended for the lot 242 Class 6 build and that is the design we will use for 72010.
Looking at the front of 71000's delta truck we can see the cast steel triangular radius arm with the load bearing surface around the pivot. This engages with a large pivot pin that locates in a bush on the front firebox stretcher. The radius arm is bolted to the front of the fabricated frame. For the suspension, a spigot locates on the top of the axlebox (via a pad) which transfers the axlebox movement to a horizontal spring beam located between the inner and outer side frames. Guides on the frames restrain the spring beam sideways with manganese steel liners welded to the guides. Spring Links drop down vertically from the ends of the spring beam to the bottom of the coil springs. The tops of the springs are restrained by the frames; therefore as the axlebox moves upwards, the spring beam moves upwards thus compressing the springs.
The axlebox is an outside bearing axlebox (Outside the wheels as opposed to the inside axleboxes fitted to the bogie and coupled wheels). The axlebox contains a pair of Timken taper roller bearings.
Returning to the top photo, we can see another important function of the trailing truck. A square section beam runs across the back of the frames. Inside are 2 pairs of coil springs separated by the shaft of the circular centring pin you can see in the photo. The centring pin is bolted to the bottom of the dragbox which is part of the locomotive's frames. As the trailing truck attempts to pivot, the centring pin will compress one of the pairs of springs. This stabilises the locomotive while running round a curve and also produces a smoother exit.
The bushes of the centring gear are lubricated by the oil box. There is also a pair of grease lubrication pipes to lubricate the centring pin.
Axle and Tyres
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For phase 1 of the pony truck construction we need to source the long lead items which are the axle and the tyres for the wheels. These components are on a 12 month order delivery. We had a problem with the first set of wheels that we ordered and these were returned to the supplier. As of January 2024, a new order has been placed with Trefoil Casting in Sheffield for a pair of wheels. The pony truck wheels are the same as the tender wheels .
Click on the images below for a larger pictures.
| Tyres | |
| The tyres are secured to the wheel by the "double snip" method shown on the drawing whereby protrusions on the tyre engage with both sides of the wheel. The tyre is heated on assembly so it expands just enough to allow one of the protrusions to slip over the wheel. Once the tyre cools and shrinks the protrusions will ngage with the sides of the wheels. On the tender, the Gibson Ring fastening system is used as the tender wheels are braked. Under heavy braking this could cause the tyre to heat and expand. 2 off tyres are required. Estimated cost £4,260 for 2 |
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| Axle | |
| The axle is 7' 6.75" long and has to be manufactured by an approved railway supplier. 1 off axle is required. Estimated cost £15,000 |
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| Axleboxes | |
| The axleboxes are common with the tender and are a casting. We will also need to procure the taper roller bearings and the internal components for the axleboxes. Steel Liners are fitted to the axlebox guides on the frames, manganese liners are welded to these liners, also to the axlebox body. 2 off axleboxes are required. Estimated cost £15,720 for two axleboxes £3,400 for 4 bearings (2 per side) £4,000 for internal components £5,000 for Manganese Steel liners £2,900 for Steel liners |
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| Frame Keeps | |
| The frame keeps are bolted onto the bottom of the axlebox guides. When removed, they allow the wheelsets and axleboxes to be removed from or inserted into the frames. Each frame keep is secured with 4 off 1" x 5-1/8" long fitted bolts 2 off are required. Estimated cost £2,000 2 off Frame Keeps £360 8 off Fitted bolts |
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Frame Assembly
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Phase 2 will address the primary components of the pony truck, namely the radial arm and the frame assembly. Like the main frames and the bogie, the frames will be assembled using fitted bolts.
| Radial Arm | |
| The radial arm is a casting. As this is a one off casting, the patternless, sand printed mould system could be used (same as the exhaust manifold). However, this will depend on quotations provided by suppliers. The radial arm will need machining after casting. The radial arm also requires a cast iron bush. This not only acts as a bearing surface for the pony truck pivot pin but also the top surface is a bearing surface for some of the weight of the rear of the locomotive. Estimated cost Radial Arm £30,000 for 1 Bush £405 for 1 |
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| Pony Truck Frame | |
The pony truck frame is made up from 5 major components which will be bolted together.
Estimated cost £25,000 for complete assembly including fasteners |
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| Spring Cover Plates | |
| 4 cover plates are fitted to the front and back faces of the rear box section beam. These allow access to the centring gear springs. Estimated cost £200 for 4 |
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| Fitted Bolts | |
| The pony truck frame was originaly assembled using cold turned rivets. We will follow the same system as for the main frames and bogie; we will use fitted bolts for the assembly. Estimated cost £780 26 off 7/8" Bolts - Radial Arm to Side Plates £360 12 off 7/8" Bolts - Radial Arm to Central I Beams £360 12 off 7/8" Bolts - Central I Beams to Rear Frame £780 26 off 7/8" Bolts - Rear Beam to Side Plates The photograph shows representative fitted bolts as used on the main frames |
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Suspension System
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Phase 3 addresses the suspension system. Unlike the Mark 1 Clans, 72010 will use the coil springing system used on 71000 - Duke of Gloucester
| Coil Springs | |
| The main suspension system is a pair of coil springs on each side of the frames. 4 off coil springs are required Estimated cost £TBD for 4 |
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| Spring Support | |
| A spring support is fitted at the bottom of each coil spring. 4 off spring supports are required Estimated cost £1,120 for 4 |
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| Spring Links | |
| A vertical spring link passes through the spring support and through the centre of the coil spring. 4 off spring supports are required Estimated cost £1,800 for 4 |
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| Spring Beams | |
| The 2 vertical spring links on each side of the frames connect with a horizontal spring beam. The spring beam is located by 4 manganese steel liner plates that are welded onto the frame plates and 4 manganese steel liners that are welded onto the spring beam. 2 off spring beams are required Estimated cost £4,500 for 2 spring beams £1,100 for 8 Mn steel liners |
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| Ball End Piece | |
| A spigot is welded to the bottom of the spring beam. Into this is fitted a ball end piece made out of spring steel. The drawing shows that a small flat must be machined onto the face of the engaging boss so that when the ball end is fitted to the spigot, air can escape. The ball end engages with an axlebox spring pad that sits on top of the axlebox. 2 off are required Estimated cost Ball End Piece £890 for 2 Spigot £570 for 2 |
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| Spring End Shoe | |
| The vertical spring lings pass through the spring beam and though these spring end shoes which sit on the top surface of the spring beam. The spring end shoes have been ordered as a bulk purchase as they are used on the bogie, driving wheels and pony truck. 4 off are required for the pony truck. Actual cost £840 for 4 |
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Centring Gear
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The pony truck centring gear is fitted in the rear beam of the frame. A substantial centring pin is fastenened to the bottom of the dragbox with the pin pointing downwards. This passes through a slot in the frame and engages in a guide fitted within a crosshead. Attached to both sides of the crosshead are spindles which pass through the box frame - the ends are supported by bushes and guides in the end plates of the frames. Each spindle runs through a pair of round section, coil springs to provide the centring force.
| Crosshead | |
| The main central component is the crosshead. This provides an engagement for the centring pin and allows the spindles to be connected. 1 is required Estimated cost £2,100 |
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| Crosshead Guide | |
| A guide/bush is fitted within the crosshead to accept the centring pin. This will be a bronze part. 1 is required Estimated cost £1,450 |
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| Spindles | |
| A spindle is attached to each side of the crosshead. A cotter secures the spindle to the crosshead 2 off are required Estimated cost Spindles £4,500 for 2 Cotters £280 for 2 |
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| Internal Sleeve | |
| Within the box section rear frame are 2 internal ribs, one each side of the crosshead. A special circular plate is welded to the rib to provide support for the sleeve and bush which will support the spindle and pass through the rib. A steel sleeve fits inside the circular plate; this has a cup on the outer end and the end of a side control spring will fit in the cup. Inside the sleeve is a bronze bush which carries the spindle 2 off are required Estimated cost Sleeve £870 for 2 Bush £1,150 for 2 |
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| Centring Spring | |
| A pair of round section, coil springs are fitted to the spindles. 4 off springs are required Estimated cost £TBD for 4 |
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| Distance Pieces | |
| Each pair of springs is separated by a distance piece. This has a slight taper working outwards from the centre. 2 off distance pieces are required Estimated cost £870 for 2 |
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| Spring Guide | |
| The outer end of the spindles pass through the side frame plates and are supported by a spring guide which is bolted to the outside of the frames. The guide is fastened to the frames with 2 standard studs and nuts and 2 special long studs fitted on diagonals. These studs allow the side control springs to be compressed as the guide is tightened down onto the frames. 2 off guides are required. Estimated cost Spring Guide £820 for 2 Studs £500 for 4 |
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Spring Guide Bush |
| The spring guide is fitted with an internal bronze bush. 2 off bushes are required Estimated cost £280 for 2 |
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| Spring Guide Sleeve | |
| Another sleeve fits the spring guide. This has a smaller cup than the other sleeve to retain the end of the spring. 2 off sleeves are required Estimated cost £705 for 2 |
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| Spring Guide Sleeve Bush | |
| The spindle runs in a bronze bush within the sleeve.. The assembly is completed with a machined collar over the end of the spindle and a nut with a 1-3/8" diameter thread. 4 off bushes are required Estimated cost Bushes £930 for 2 Collar £270 for 2 |
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| Rubbing Plate | |
| A manganese steel rubbing plate with lubrication grooves is welded to the top surface of the bottom plate of the hind beam of the frame. The crosshead will slide on this. 1 off is required Estimated cost £350 for 1 |
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| Cover Plate | |
| A folded cover plate is fastened to the top of rear beam to provide some protection from ash. 1 off is required Estimated cost £700 for 1 |
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Other Items
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A number of miscellaneous items are need to complete the pony truck.
| Bolster Cups | |
| The pony truck bolsters are fitted to the gussets on the rear of the frame extensions. The domes on the bolsters engage with these bolster cups which are free to "float" on the oval alloy pads fastened to the pony truck frame. As the pony truck pivots around the pivot pin at the front of the frame, the back end must be able to move. 2 bolster cups are required Estimated cost £800 for 2 |
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| Bolster Pads | |
| The bolster cups float on oval shaped, bronze pads fastened to the top of the rear frame. 2 bolster pads are required Estimated cost £1,200 for 2 |
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