Srd Commando Crank balance weights

Status
Not open for further replies.
andyd

andyd

VIP MEMBER
Joined
Jan 23, 2012
Messages
42
Country flag
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
 
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
 
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.
 
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?
Click to expand...

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
 
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
 
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
Click to expand...

Sadist !
 
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.
 
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.
 
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.
 
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.
 
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
Click to expand...
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.
 
Status
Not open for further replies.
Back
Top