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DIY crankshaft balancing with an oscilloscope 10

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NDimitrov

NDimitrov

Electrical
Sep 13, 2023
12
Hi all,
since I cannot find a local place to balance my crank I started to make a balancer stand using ADXL335 accelerometers and a QRD1114 looking at a reflective tape for triggering .
Finding the heavy spot using the acceleration plots is the easy bits .
I can't make the connection between acceleration and gr/cm imbalance .
Could somebody shed some light on that topic ?

Kind regards
Nikolai
 
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single plane or 2 plane balancing? Assuming the latter. All measurements are complex, that is you know the phase relative to a fixed marker on the crank.
I1=initial vibration at plane 1
I2=initial vibration at plane 2
add a trial oob TB1 at plane 1, vibrations are TM1_1 and TM1_2
take it off and put it on plane 2 at the same angle
vibrations are TM2_1 and TM2_2
so
TM1_1=I1+TB1*IV11
TM1_2=I2+TB1*IV12
TM2_1=I1+TB2*IV21
TM2_2=I2+TB2*IV22

where IV is the influence vector. TB1=TB2=TB in this procedure, and IV21=IV12 by reciprocity. So work out IV11, IV12, and IV22, (eg IV11=(TM1_1-I1)/TB1) and solve for a final balance at each plane to get zero vibration 0=I1+FB1*IV11 etc



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
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An inline or opposed engine with 4 or more cylinders ?
 
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The crankshaft in question is for an inline 5 cylinder diesel .
I expect to see 5 distinct disbalances, and when the time comes to get it in spec I'll try adding play-dough until everything is balanced .
 
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It is usually only necessary to balance a properly designed 4 or 5 or 6 or 8 cylinder engine at each end. If the crank is wildly oob then you can drill some of the other webs, but you are removing more mass for a given effect, and affecting the counterbalancing strategy slightly. You should always single plane balance the harmonic damper (front pulley) and the flywheel separately before assembly to the crank.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
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There is quite a difference in weight between the original and forged rods that'll go in . And while I'm at it I'd like to also remove as much unneccessary weight as possible . Much like David Vizard shows in his videos .
And to get back on topic .
On a 5 cyl crank I expect number 3 journal to show up equally on both accelerometers. With number4 and 5 having more impact on the rear plane and vice versa .So number 3 should show up as static .
As far as removing the weight I'll aim to find the heavy spot and remove more material from the edges of the counterweights, knife edging the crank .

A big thank you for the productive conversation !
 
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IMG_0724.v01_tyj8cs.jpg

IMG_0725.v01_jvwad3.jpg

That's what it looks like . Nothing fancy . For the drive I'll use a 9500 rpm angle grinder with some electronics to limit the rpm .
 
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I suspect your fixture design is going to give you some trouble due to a lack of in-plane stiffness. If you test as-is you'll find this out.

What you really want is a LOT of stiffness in the vertical and longitudinal planes of the crank - this is why most commercial balancers use solid plate for the idler mounts. This setup may work at some level, but you're likely to lose a lot of sensitivity because your in-plane mount stiffness levels are not going to be very high using extrusions in bending like that.
 
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I used to balance engines hanging from chains, I don't think a soft balancing machine is necessarily bad.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
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Its better than nothing, its called use what you have. Is it located high for a reason, like larger diameter counter weights?
Very good construction, you put some work into it. Its nice to see some DIY things like this.
 
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I'd like to balance the flywheel as well . And give me more room for the drive motor . Those ADXL335 acclelrometer don't have any crosstalk between the axes , so I don't expect to see anything from deflection in the Z axis . Might end up putting a steel wire and tensioner to reduce deflection .
 
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balance the crank alone, 2 planes, and the harmonic balancer and the flywheel, 1 plane, and then the assembly 2 planes.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
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NDimitrov have you made your own bob weight carriers or did you buy off-the-shelf? Good luck with your project! Seeing your results will be very interesting.
 
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I'll have to make them myself . Seeing the bobweight will have to be more than 1kg I'll be making them from steel.
Calculating the acceleration of the piston in different angles with this calculator I don't see 50 % as the ideal balance factor . Something around 64% seems better at equalizing the forces . Any thoughts on that ? How would you guys go about selecting the balance factor ?
 
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64 seems very high, but I could believe an I5 is special. One proper way to do it is FEA looking at main bearing pressures, but in the olden days it was done by hand.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
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If the reciprocating masses are in simple harmonic motion, wouldn't 100% bobweight eliminate the reciprocating imbalance and create a new linear imbalance of the same magnitude at 90* to the cylinder axis? That would suggest an ideal bobweight closer to 50%.

je suis charlie
 
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One approach, which is not mass efficient, is to treat the crank as a series of single cylinders, and then work out the balance factors (and hence the webs) for a single cylinder. This ignores the dynamics of the crankshaft. We did this and ended up with a good strong revvable crank, at a 2 kg weight penalty.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
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