Srd Commando Crank balance weights

andyd · Nov 16, 2019

Hi guys,
I have four 850 cranks in pieces that were not marked before they were separated. I was told the crank pieces are still in matched sets but I have no idea which sides to mount the journals. All the cheeks are drilled differently and are not central to the journal centre axis.
Any chance anyone has dimensions of steel balance weights so that I can then machine up a couple of weights and fit to the big end journals ?
I can do a final weight adjustment once I have the basic weights, but rather than try to calculate the weight of a piece of steel 1 inch wide with a 1.75 inch dia hole in it i thought someone may have the approximate size to get me started. Or is it a simple calculation?
I can turn down a bar of steel and split it to fit to the beg end journals, just need a starting point for dimensions. The cranks are for std Commandos so will be happy with the std dry balance factor.
Or, is the above not needed and I am missing some simple method to reassemble the crank correctly? .
Cheers,
Andy

comnoz · Nov 16, 2019

The simpler way to get a static balance is to hang the bob weights on a string through the center of the oil hole in the rod journal. Tie some small fishing string to a needle and center it in the top hole. When you have static balance correct your string will be centered in the lower hole. ie- the oil holes with be vertically in line.

Often you can tell the correct orientation of the flywheel by looking for stains on the surfaces of the joint -if they haven't been cleaned too well.

If you want to make a bob weight for static balancing then weigh out the correct amount of solder and wrap it around the journal. Or use stick on wheel weights. Jim

acotrel · Nov 21, 2019

I think you are crazy. Start from the beginning with a set which has not been too butchered, reassemble the crank then put it between centres and check the alignment. After that, rebalance it from scratch.

mdt-son · Nov 21, 2019

andyd said:
Any chance anyone has dimensions of steel balance weights so that I can then machine up a couple of weights and fit to the big end journals ?
I can do a final weight adjustment once I have the basic weights, but rather than try to calculate the weight of a piece of steel 1 inch wide with a 1.75 inch dia hole in it i thought someone may have the approximate size to get me started. Or is it a simple calculation?

Hi Andy,
It's quite simple. The balance should have the weight of one complete piston + 1/3rd of the conrod weight. Weight of a tube (if we consider the two balance halfs to form a unified part) is
M = density * pi/4 * (D**2 -d**2) * Length, where d = 1.75 in. Density is 7850 kg/m3. For a given length you may calculate the required D. Due to the gap of the split cylinder, you need to increase D slightly, say by 5%. The weight of the "gap" is
m = density * (D - d)*Length*gap.
For a given gap, say 5mm, you may calculate m. Then, find the new D from (M-m). It's an iterative process but a few rounds will give you a useable figure.

Regards,
Knut

lcrken · Nov 21, 2019

Like Jim said, sometimes you can figure it out from the oil stains. The usual practice when disassembling a crank is to mark the halves and the center with punch marks so you can match them up properly when assembling. Looks like the guy who did it didn't know that.

Ken

acotrel · Nov 21, 2019

mdt-son said:
Hi Andy,
It's quite simple. The balance should have the weight of one complete piston + 1/3rd of the conrod weight. Weight of a tube (if we consider the two balance halfs to form a unified part) is
M = density * pi/4 * (D**2 -d**2) * Length, where d = 1.75 in. Density is 7850 kg/m3. For a given length you may calculate the required D. Due to the gap of the split cylinder, you need to increase D slightly, say by 5%. The weight of the "gap" is
m = density * (D - d)*Length*gap.
For a given gap, say 5mm, you may calculate m. Then, find the new D from (M-m). It's an iterative process but a few rounds will give you a useable figure.

Regards,
Knut

Sadist !

kommando · Nov 21, 2019

I used the oil stains to confirm the flywheel was the right way round on an unmarked flywheel not long ago, with 4 cranks to do plus a weight for seeing how well your assembled cranks statically balance you could do it over a day.

johnm · Nov 21, 2019

I’m a little hesitant post here because while I’m familiar with the concepts of static balance I’m not following some of the discussion above.

But concerning the mixing up of crank parts I recently had to replace My crank due to a crack at the drive end. The “ original condition “ replacement crank turned up with a strange collection of holes and worn big end journals. So it needed to be refurbished. When it was dynamically balance it was considerable out between the left and right ends. So if you are possibly mixing up parts I would have it dynamically balanced.

acotrel · Nov 23, 2019

If you are having your crank dynamically balanced, be very careful about whom you get to do it. Some of the car guys are complete idiots. The rocking couple (secondary balance ) is usually not an issue, but they can make it one and stuff your crank.

comnoz · Nov 23, 2019

Ah, Eliminating the rocking couple is what dynamic balancing is all about....

If you don't care about the rocking couple then just do a static balance.

Joe Schlaberdowski · Nov 24, 2019

andyd said:
Hi guys,
I have four 850 cranks in pieces that were not marked before they were separated. I was told the crank pieces are still in matched sets but I have no idea which sides to mount the journals. All the cheeks are drilled differently and are not central to the journal centre axis.
Any chance anyone has dimensions of steel balance weights so that I can then machine up a couple of weights and fit to the big end journals ?
I can do a final weight adjustment once I have the basic weights, but rather than try to calculate the weight of a piece of steel 1 inch wide with a 1.75 inch dia hole in it i thought someone may have the approximate size to get me started. Or is it a simple calculation?
I can turn down a bar of steel and split it to fit to the beg end journals, just need a starting point for dimensions. The cranks are for std Commandos so will be happy with the std dry balance factor.
Or, is the above not needed and I am missing some simple method to reassemble the crank correctly? .
Cheers,
Andy

From my understanding, steel is homogeneous. Of a certain composition a particular volume of it from anywhere it is taken will weigh the same. Therefore, accurately measure the steel you have. The hole that is drilled is a cylinder (easy geometry calculation) to be subtracted from the calculation of the volume of your bar of steel (lxwxh). The drill hole remains the same so the only variable, I'm guessing, is the length of the steel bar you slice off. But sometimes steel bars are very irregular along their sides, having been sheared off. So I'd check that. What remains is multiply steel bar volume times density of your steel (easy table look up) leaving the steel bar length as X to calculate against the desired weight. Gives you a fairly accurate weight you can start with.

Srd Commando Crank balance weights

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