Stuart Grasshopper – beam pivot supports; puzzled

[iconParticipant @icon]

I’m making the Stuart Grasshopper/Half Beam engine. I’m currently machining the brass beam pivots. The plans call for the 1/2″ diameter pivot section to be 15/32″ long, but the beam is 1/2″ thick where the pivot runs. That means that when the other part of the support is screwed home, it will clamp the beam. Am I missing something? Has anyone made this engine and can tell me where I’m screwing up? Surely the pivot want to be a few thou over 0.5″?

[SillyOldDufferModerator @sillyoldduffer]

Be really good if you can manage a picture of the plan or engine itself,  I’m not familiar with that engine.

Assume it’s this one, but do you mean the pivots marked in Red, or the one in Blue?

In all cases, nothing should be tightened enough to clamp the beam, so the answer may simply be to super-glue or Loctite all the nuts in position just short of clamp point.  The glue stops them coming loose due to vibration and falling off.

Not clear in my picture how the pivot ringed in Blue is secured on the other side, and I guess this is the one causing concern.  Same principle though – don’t over tighten it.

My limited experience of engine building suggests models work better if all the moving parts are slightly sloppy.  I found the hard way that making them with high accuracy is likely to increase friction and binding.  Problem is the way the beam translates linear piston strokes into a sinusoidal turning moment on the flywheel is a tricky design compromise. Getting a good answer to this difficulty is one of the things that put James Watt in the genius class!

In a model the easiest way to allow for the mechanical translation being slightly ‘off’, is to give the motion a bit of elbow room by allowing the pivots a tad wobble space.  Not too much because ‘running in’ tends to open up and burnish any bearings with tight spots so they work well and last.  Too much slop increases wear and tear, though a model might never be run long or hard enough for wear to matter.

Dave

 

[JasonBModerator @jasonb]

It’s the only one of their 2″ x 1″ cylinder engines I have not made.

I wonder if it is a case that has cropped up before of copy and pasting parts from other engine drawings. The brass pivots need to be able to rotate so why is there a locking screw in the top of the beam, it is used on the actual beam engine to lock the steel pin that is used on that design.

I would not drill and tap the hole in the beam.

Check the dimension at the other end of the links and adjust either beam boss width or the central portion of the pivot ,myself I would go with a 1/2″ beam width and increase the pivot to say 0.502″ making the diameters a good rattle free fit .

I would also suggest that you don’t drill the 4BA holes for the link arms until you have screwed the two halves of the pivot together then you can make sure they line up radially and are the correct 1.75″ apart and equally spaced either side of the beam’s ctr line

[iconParticipant @icon]

Thanks Dave – it is indeed the one in blue, and Jason has provided the drawing above. Below is the drawing for the beam. It seems like an obvious solution to make the pivot section fractionally longer than the beam is wide at the pivot point – 33/64″ as Stuart would probably call it. The engine has been around donkey’s years and the plans have been through any number of revisions, so it seems a bit odd that it’s showing 15/32″. Unless I’m being an idiot?

 

 

I think Stuart might have an oil hole in mind for the hole on top. Of course I’ve already made the offending bit so that’s destined for the scrap bin.

[JasonBModerator @jasonb]

A 64th is too much, although it may run freely who wants an engine that knocks. In the old days you would have seen something like “1/2″ Full” maybe with the beam as “1/2″ tight”

Very few revisions on Stuart drawings over the years, yours look the later set but they were just redrawn not altered and many parts copied and pasted from one engine to another. There are the odd errata sheets that were issued but if a casting set changes hands a few times they can get lost and in some cases the correction causes other errors.

As said the 3/32″ hole is most likely a carry over from the Stuart beam the pin locked the beam and pivot together so that the steel pivot rotated in the bearings and did not rotate in the beam. Too big for an oil hole unless you want oil running all over the engine. You could turn the beam upside down and add a smaller 1.2mm oil hole with a countersunk recess or open the top up a bit and thread for an oil pot and have a small hole in the pot.

[Martin ConnellyParticipant @martinconnelly55370]

My solution is to only drill one of the 4BA holes in either item 90 or 91 until assembly has been checked. I found that the 2BA thread on item 90 was a bit tight as I did not undercut the shoulder. This allowed the 2BA thread in item to be counterbored a bit to allow the parts to get closer and when I has satisfactory clearance mark the position of the undrilled hole for the 4BA thread. When drilled and tapped like this the two arms item 93 will hold the parts in position with just the right amount of clearance.

Martin C

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