# Concentric bolt circles?

• posted

I need to make a couple of adapter plates for the new 56HP Yanmar to fit the CV joint to the prop shaft. The gear box flange has four M10x1.5 threaded holes on a 78mm pitch circle and the CV joint has six M10x1.5 holes on a 93mm pitch circle.

At anything other than a 15 degree offset between the two circles the holes interfere. At 15 degrees I have 3mm between the holes. I am limited to

22mm in thickness so I can't drill blind holes on either side. Is 3mm between holes sufficient or is there another way to lay out the bolt circles?
• posted

Hi Glenn,

Surely the least metal you can have between the holes is 5 mm? The outer edge of the holes on the 78 mm circle will be at a radius of 83 mm, and the inner edge of those on the 93 mm circle will be at a radius of 88 mm. So I figure there's a minimum of 5 mm between them. Someone correct me if I'm wrong here...

I reckon 5 mm will be okay, but I'd be inclined to make the adaptor plate the full 22 mm thick.

There is a mathematical way of figuring out an optimum hole spacing, but right now I can't think of a simple mathematical way. But I'll have a go if you like.

Best wishes,

Chris

• posted

Having thought about it a little more, I think 15 degrees may very well be the optimum offset. I don't think there is much to be gained by doing a load of maths :-).

Chris

• posted

It's customary to specify bolt circles as the diameter, not radius. It's a tight fit.

I have no answers for the OP, but I will say this. It might be better to use only four of the six mount holes rather than compromise all four of the other set.

• posted

If that's the case, it's a very tight fit. Mike's idea of only omitting a couple of bolts could be a good one. Perhaps other people can add their thoughts?

Chris

• posted

Without looking at the flanges, it's hard to make any recommendations, but I'd be tempted to plug the holes, weld an "extension ring" (if it's needed) on the small flange to match its outside diameter with the larger one and redrill the holes, or make a new flange (although the thought of creating splines in the inside of the new flange would give me pause...).

Peter

• posted

3 mm will be pushing it. Have you looked at LHSHCS? (Low-Head Socket- Head Cap Screws). Check:
Head height is only 6 mm, leaving you 15.5 mm of material thickness without a washer (0.5 cbore clearance), subtract another 1.25 mm if you use a serrated cone (bellville style) lockwasher.

Anthony

• posted

"Mike Young" wrote

I got a look at a "stock adaptor for a 100mm PCD adaptor. Only 4 holes are drilled for the 6 hole side so that looks like the solution.

Never realized how much I have saved with my mill and lathe. Took the drawings down to a local machine shop and got a price of \$240 each for the adaptors. They are just a 4.5" diameter round 7/8" thick bored 1.75" with a 3"x 1/4" relief on one face and a 1/8"x1/8" spigot around the bore on the other. I can turn them both out in half a day on my antique 10K. In making parts for Rutu I figure I have paid for both the mill and the lathe 5 or 6 times. Of course, I bill my time at \$.01/hour after the satisfaction discount. :-)

• posted

I can't imagine why it would be a problem with a reasonably strong and malleable material (e.g., mild steel or 300 series SS). Consider that the wall between the holes at the thinnest point is only 0.5mm less than the wall of a standard M10 nut.

Is there a specific failure mode you're concerned about?

Ned Simmons

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