Practical load calculation method
Practical load calculation method
(OP)
Hello everyone,
I have a special application with a spindle that is planing wood. I'm looking for a way to calculate the load on bearings in operation. Since there are too many variables, it's almost impossible to have a good approximation by theorical calculation. Thus, I'm looking for a practical way to measure the load. I thought I could have a good value by measuring the deflection of the shaft in operation and then, evaluate the bearing load. My concern is about the way to calculate that value. Should I only compute the bearing deflection or the shaft deflection or both. The spindle has a pair of angular contact bearing 15 deg 100mm-150mm ligh preload at each end. Is there a general thumb rule for that? Will it be a good approximation to only calculate the bearing radial deflection?
Thanks,
I have a special application with a spindle that is planing wood. I'm looking for a way to calculate the load on bearings in operation. Since there are too many variables, it's almost impossible to have a good approximation by theorical calculation. Thus, I'm looking for a practical way to measure the load. I thought I could have a good value by measuring the deflection of the shaft in operation and then, evaluate the bearing load. My concern is about the way to calculate that value. Should I only compute the bearing deflection or the shaft deflection or both. The spindle has a pair of angular contact bearing 15 deg 100mm-150mm ligh preload at each end. Is there a general thumb rule for that? Will it be a good approximation to only calculate the bearing radial deflection?
Thanks,





RE: Practical load calculation method
I think that deflection measurement is way too uncertain. If I understand right, the planing means that you have a mostly tangential force on your tool and that it will be transferred directly to the spindle shaft.
A simple method would then be to measure active power used by the motor when idling and when it is working. The difference is the power used to remove chips from wood.
Using this information and knowing spindle speed as well as tool diameter will make a fairly accurate calculation of the actual force possible. At least with around +/-30 percent accuracy - and that is probably the best you can get without mounting the assembly on force transducers.
Gunnar Englund
www.gke.org
RE: Practical load calculation method
RE: Practical load calculation method
I have done these measurements/calculations on high-speed spindles where active power can be read directly from the drive. The air gaps will not mean any increased impact - rather the opposite. The tool will start hitting the upper corner and chip that away (low power), the next chip will need a little more power and so on until there is a series of chips with same power until end of plank. There are no extreme forces when hitting the first corner so you do not need to take that into account. The direction of the force will be somewhat different. But since these initial chips are small, you do not really need to bother about them.
Measuring shaft deflection or bearing deflection still seems (to me) to be much more error-prone. Has anyone done that with success? And calculated forces with any accuracy? I think that we are talking high-speed here, not a few hundred RPM.
Gunnar Englund
www.gke.org
RE: Practical load calculation method
Here is maybe a compromise. With the spindle running maximum, apply also the maximum operational load and somehow and record it. Turn the thing off and apply the same load but this time in static. Measure the deflection before and after the load.
RE: Practical load calculation method
Apply same load, but static. Would you do that by pressing a plank against the still-standing spindle? In that case, wouldn't the force used to press plank against spindle be the answer?
Or have I got the configuration wrong?
Gunnar Englund
www.gke.org
RE: Practical load calculation method
rnd2 I think your idea can work. I should try it in a few weeks and let you guys know.
Thanks,
RE: Practical load calculation method
RE: Practical load calculation method
Looking forward to your results.
Gunnar Englund
www.gke.org
RE: Practical load calculation method
-Using a dial indicator with the right range. Too precise leads to a difficulty to take the readings. Not enough leads to the incapacity to take the readings.
-Forecast a stand to hold the indicator. In fact, the vibration on the machine false the readings.
-For mor precision, a good tip would be to film the indicator in operation. Therefore, it will be easier to exactly see what is the measurement.
-Try different angles on the shaft to make sure you have the optimum deflection angle.
This method works good and I would recommand it to anyone who need to get the loads on his machine and have no other to do it.
RE: Practical load calculation method
Interesting to have your feedback.
I have been using LVDTs for similar measurements. They usually come with electronic modules where you can filter out the vibrations and get a steady mean value. They also allow you to select ranges - often between 0.01 mm to 10 mm - and that should be sufficient in most cases.
Gunnar Englund
www.gke.org
RE: Practical load calculation method
RE: Practical load calculation method
Gunnar Englund
www.gke.org