Sweet Pea Valve Gear

My poor little Sweet Pea, Ivy, has been a tad poorly of late. She has been progressively losing power for a while, and when I fired her up for the first run of 2016, back at Easter, she could barely haul herself round the track at Abbeydale, let alone passengers! Dismayed at this, I set about finding the cause...

Sweet Peas are designed to utilise Hackworth valve gear to transmit motion from the axles into correctly timed motion for the valves, which let the steam into each end of the cylinders. This is done by using a valve rod to push a sliding block back and forth over the cylinder ports: first letting steam into one end whilst exhausting the other, then switching to the other end to repeat the process there. Nice and simple.

Except, not THAT simple. The exact positioning and timing of this valve block is fairly critical, and errors in its position can seriously affect the engines performance. This, therefore, was my first port of call. I took the covers off the valve chests, and checked the timing. It seemed in order, so I sealed it up again and ran it. No joy. Very erratic movement, and a lot of steam was being lost up the chimney without ever touching the pistons.

That suggested to me that the valve blocks themselves might be worn. With the assistance of a chap named Steve Addy, I removed the blocks and did a bit of measurement on its dimensions. It turns out that whoever made the valves made a bit of a hash of it, and the faces were chipped and the exhaust ports in the underside were so oversized that they spanned the two inlet ports! This doesn't sound too bad until you realize that effectively, both ends of the cylinder can exhaust out of the chimney at the same time, which is a massive waste of steam. This would not do, so it was off to Steve's workshop we went to make use of his milling machine. There, we made a pair of correctly dimensioned cast iron valve blocks, with beautiful faces polished up on his surface grinder.

We put these back in, and put Ivy in steam again. There was a little improvement, but something was still amiss. When the regulator was opened, instead of motion there was a rushing sound from the chimney. This sounded very much like steam was getting past the valves, and straight out of the exhaust! Not conducive to movement. After a bit of head scratching, I found an article online that suggested that as valve gear wears, the slack that gets into the system results in less valve travel. This means that the valve block is wearing a path in the port face of the cylinder that gets progressively shorter, leaving a ramp at each end of its travel. When the valve gear is tightened up (as I have recently done to Ivy) and full valve travel is restored, the block rides up onto these ramps and pulls away from the port face, letting steam past. That sounded very much like Ivy's problem, so I went to check the port facing.

On removing the valve chest covers and the valves, it was immediately apparent that this groove had indeed been formed, and the ramps were there as suggested:

There was nothing for it but to remove the valve chests, take out the studding and try to flatten and smooth the port faces of the cylinders! After a couple of days work with a diamond lap attached to a nice, stiff piece of flat bar, I had a wonderful smooth surface to both cylinder faces:

The new valve blocks slid smoothly back and forth along it, and it all looked rather promising.

I reassembled the valve chests, retimed the valve events and put the covers back on. With a good squirt of oil in each chest (cast iron does like a bit of steam oil!) Steve had the good idea of connecting a compressor to the engine to see if it would run nicely on air. After a bit of bodging to attach it to the boiler, we put 50psi in her, put the reverser in full forwards notch and tentatively opened the regulator a crack...

...and off she shot! There was even, according to Steve, a bit of wheelspin! We ran her back and forth a bit, testing her out, before getting excited and wheeling her out onto the main track. With the compressor sat behind her on a riding car, acting as a sort of extra reservoir, she managed half a lap before running out of air. She was even happy to notch back, both in forward and reverse! Theoretically, if she behaves this well on compressed air, she will be even better with a fire in her, as steam expands more effectively than air.

I am very pleased with the results of these few weeks of experimentation, and can't wait to put her in steam on Monday! My thanks go to Steve Addy, for his encouragement, assistance and hard work in getting poor Ivy back on the rails.


It worked! Ivy is now back up and running like she never has before! So well, that I am thinking it may be time to try her down on the ground level track at Abbeydale again. I think she will make it without too much trouble now!