balanced crank? do the rods need doing with it???

Boosted LS1 · Post by **Boosted LS1** » Wed Mar 07, 2007 4:52 pm

It depends on the jig you made. I put a bobbin through each end of the rod which then sits more or less horizontally and facing away from me. Pressed through the centre of the bobbin is a pin maybe 8mm o/d. So, each rod end has a pin through it. One end is on the scales the other on the stands. Each pin rests on 2 vertical stands with a semi-circular groove cut across them. This keeps the rods straight and locates the pins. I mark the bobbins for orientation in case they have differing density etc.

I take say 10 readings for each end of the rod and so long as 7/8 of them are the same I know it's measuring accurately. If the rod moves sideways a smidgen it can screw it all up hence the grooved stands. Also, scales may weigh to 1/10th of a gram but they will only be accurate to 1/10 gm so you may as well work to 1/2 a gram. That's still extremely good. I know my house/floor moves from traffic outside just from watching the scales!

Boosted.

kiwicar · Post by **kiwicar** » Wed Mar 07, 2007 5:13 pm

Hi Boosted
Made my jig by putting a gudgeon pin through the little end and making a support of two beams comming out from an ally block supported at the other end like a small set of parrellel bars (out of 10mm tube), I sat the gudgion pin on these and the big end on the scales then set the height of the "bars" so the center line of the rod was horizontal when the big end was on the scale. I got a huge spread of results on the scales so stuck my thumb in my mouth and sulked

I was thinking of making a more solid version of this but I am unsure if it will be any better.
Mike

Boosted LS1 · Post by **Boosted LS1** » Wed Mar 07, 2007 7:16 pm

The big end bobbin needs to be on pins as well, small pins are best. It gives a lot more precision. Best with a groove in all the supports then you know the rod hasn't moved sideways a smidgen. You don't want the bobbons moving sideways in the rods either. Also, the gudgeon pin may be heavier on one side which is why I mark my bobbins/pins. This ensures a degree of consistency. Imagine looking down on the rod from above. If it doesn't stay facing in a true fore/aft position then weight will be transferred from the scale end of the rod to the end resting on the pivot because the rod will have moved in a horizontal arc. Same applies if the bobbins aren't centred. My bobbins are aluminium and have two 'flats' on them for ease of fitment. Have I explained it well enough?

Boosted.

kiwicar · Post by **kiwicar** » Thu Mar 08, 2007 9:49 am

Hi Boosted
I shall have to re read your explanation tonight but I think I get the picture

Many thanks
Mike

bill shurvinton · Post by **bill shurvinton** » Thu Mar 08, 2007 11:44 am

Vibration free are worth going to if you are building a full on motor as they do balance the entire rotating assembly. a few ££ more, but I would use them on an engine I expected to use in anger.