Hackworth valve gear question

[Benjamin Maggi]

My "Sweet William" engine I am building has Hackworth valve gear. The slide boxes mount on the end of a round shaft (called the "weighshaft"), and as the engine goes down the track the valve gear would be connected to a slide in these slide boxes that would rotate the boxes a little bit in each direction. The plans call for attaching two brackets (one per side) to the chassis which stick up above the sides of the frame and then mounting a tube between the brackets with bushings mounted at each end of the tube, and the weighshaft would run through this tube and inside the bushings. .

Hackworth valve gear bracket.jpg (8.22 KiB) Viewed 7738 times

In the diagram above, the green parts are the brackets, the red part is the tube, the blue part is the shaft where the slide boxes would mount to the end of (not shown), and the yellow part are the bushings. Is there any reason to not just mount bushings in the brackets (green parts) and omit the red tube altogether? Do I need the tube which has the weighshaft inside of it? It is extra work to machine the tubing, installing bushings in the ends, and secure it to the brackets... and I don't see what the benefit is.

Can anyone with Hackworth valve gear comment on this?

[Bob D.]

Two benefits of the tube. It insures alignment of the bushings to prevent binding. If the bracket mounting is spot on then you would be ok. Another tube plus is it cuts the bushing exposure to grit in half.

Bob

[Steve Goodbody]

Hi Benjamin,

With Hackworth gear the reversing lever is connected to the weighshaft and the slotted links are held in a fixed position by the reversing lever. Hence the slotted links should not rotate when the engine is running as a result of the movement of the die-block or forces from the valve rod.

Hackworth is a nice and simple valve gear but is very susceptible to timing issues when the geometry changes, for example due to vertical movement of the driving axle or from inadvertent rotation of the weighshaft.

I strongly suspect that the reason for the in-tube design as shown in your drawing is to get the weighshaft bearings as close to the slotted links as possible to provide rigidity and strength to the whole assembly to prevent any unwanted movement of the slide block. If you omit the outer tube then I suspect you'll need to modify and strengthen the brackets (and possibly increase the weighshaft diameter) in order to prevent wear and unwanted movement due the cantilevered forces between the brackets and the slotted links.

You can see a Sweet Pea (the smaller version of Sweet William) running in this video, this may be of help to see what's going on:
https://www.youtube.com/watch?v=SOwc9sJxuyo

Best regards
Steve Goodbody

[Bill Shields]

Steve provides a good suggestion with respect to the vertical axle motion.

Strongly suggest that you not set your valve events UNTIL you have a full load on your suspension and / or lock your suspension so that the drivers cannot move vertically..

otherwise you will be re-timing things later on.

[Fred_V]

on my SW I used rubber blocks instead of springs on the rear axle boxes. This kept the valve timing correct at the expense of the engine rocking sideways a bit on rough track. It was not a problem though.

[Benjamin Maggi]

Thank you all for your input! I appreciate it.

[dbyam]

Ben, I suggest you use Tapered Pins in place of the roll pins shown in the plans, to fasten the shaft to the slide boxes. I used the roll pins and detected some twisting moments in the blocks when I ran my engine on air.

[Fred_V]

Ben, I suggest you use Tapered Pins in place of the roll pins shown in the plans, to fasten the shaft to the slide boxes. I used the roll pins and detected some twisting moments in the blocks when I ran my engine on air.

the Allen Chloe calls for 1/16" dia pins to hole the crossover lever on it's shaft for the valve gear. It is grossly inadequate for the job. I changed them to taper pins and even had to thread the small end with a nut to keep them in place.