TORQUE CALC-Can anyone help?

I am busy with a proposal for a customer, part of which requires an hydraulic motor to drive a 4 wheeled "trolley", weighing 10 000 Kg. The trolley will travel, at very slow speed, forwards and backwards over a distance of 15 metres on the level. The track is rail line. The contact diameter of the wheels is 280mm. The motor will drive 1 axle via chain and sprocket arrangement. The hydraulic power pack will supply 11 litres/min at 200 bar pressure. How do I calculate the torque rating of the hydraulic motor, if I start at a 1:1 sprocket ratio? Obviously the sprocket ratio can be changed to reduce the torque required, but will also reduce the (non critical) speed.

If anyone can help it would be greatly appreciated. If I can swing this job it will go a long way to erasing the pain of the shafting I posted about recently.

Best regards
Tony

BACH

Does the power pack have to share its oil delivery across other tasks?
If not, has it been sized for this role, or is it just one which happens to be kicking around?

I ask this because from the info given, it's not possible to work out the 'demand side' of the problem, in other words, what torque is required at the trolley wheel in order to accelerate the load while offsetting rolling resistance.

It can only be solved in the form given as a 'supply side' problem, in other words, assume the entire hydraulic output is necessary to move the load, decide on a trolley speed and chain ratio, convert this to motor speed, and size the motor (in terms of litres per rev) so that at that speed it absorbs something like 80% of the available oil flow.

The motor torque which results from this will be the maximum torque that power pack could 'keep up with' - but it might be way overkill

Is there any way you can establish the rolling resistance of the laden trolley ahead of time (and multiply it by 2 or more to accomodate corrosion, maintenance issues with bearings etc etc)

Too much information for your core question: how much torque to one axle to establish motion in your senario. The start torque is the greatest hurdle.

You have plenty of "power" at 11 L/min @ 200 bar to move this load without wheels, straight up if you wish.

Does "rail line" mean typical full size railroad track and flanged 280mm hard wheels or something else?

Thanks Troup and Dave for your responses.

Yes, full size railroad track and hard flanged wheels. I am looking at a catalogue which underlines what you are saying about power availability of the hydraulic system.

The unit is a furnace charging trolley. The hydraulic power pack will supply 3 distinct actions, starting at the ore loading position 15m from the furnace. A. a pair of cylinders which will lower and raise the charging "chute", B. the hydraulic motor which will drive the unit from the "ore" loading position to the furnace charging position, C. a multi-stage cylinder which will eject the "ore" from the loading "chute" into the furnace. The 3 functions need not overlap.

Speed of travel should be around 50 metres/min, therefore wheel rpm is only 57.

The power pack has been sized (preliminary) for the cylinders, now I am grappling with the motor drive issue to see whether the various items of equipment are "compatible".

I am reasonably familiar with the cylinder side of things, but have never dealt with hyd motors before.

What would the basic calculation be to give me an idea of the torque required to start the trolley moving, so that I have somewhere to start from when looking at the multitude of catalogued motors?
More questions
- I assume that the rotation of hyd motors is reversible??
- does an hyd motor have a minimum rpm in order to work efficiently? The specs I am looking at have max rpm at 4000, I was hoping not to need a lot of reduction between the motor and axle.

Thanks for your interest, it is greatly appreciated
Best regards
Tony

BACH said:

.,..

What would the basic calculation be to give me an idea of the torque required to start the trolley moving...

More questions
- I assume that the rotation of hyd motors is reversible??
- does an hyd motor have a minimum rpm in order to work efficiently? The specs I am looking at have max rpm at 4000, I was hoping not to need a lot of reduction between the motor and axle.

Click to expand...

Tony
The basic calculation is F=m*a
You know the mass, so you'll have to take a stab at the acceleration time, then work out acceleration as v/t
where v is final speed, t is time to accelerate from zero to that speed, plug that in to F=m*a to get the accelerating force, ADD IN the resistance force which you will have to estimate - imagine how hard you would have to pull on a rope to tow it along at a constant speed.

The accelerating force is a variable, down to a very low value if you have all the time in the world to get it up to speed, but the resisting force is relatively constant. (Friction, deformation of the wheel and rail, etc.)

Hydraulic motors are indeed reversible. They also deliver (almost) maximum torque at zero revs, which is handy in this sort of application.

Minimum speed depends a great deal on motor design. If you don't want lots of gearing, best to go for a slow-speed motor, either a wheel motor or a standalone. Perhaps a geroler type (see Google
for basic underlying fluid displacement principle see http://en.wikipedia.org/wiki/Gerotor
However the geroler incorporates an additional orbital wobble action, which provides a certain amount of inherent gearing-down.

It's pretty expensive and inefficient to run a high-speed motor at low speed, especially if you're dropping under a couple of hundred rpm, depending on size and design.

All the best with your project, sounds interesting.

Thanks Troup for your help.
I'm working then with a mass of 10000kg (100000N?)
Say accel to 50m/min in 10 seconds therefore 0.0833 m/s^2
therefore F= 100000N x 0.0833 = 8333Nm plus rolling resistance at 2000Nm =10333Nm
How do I get to ft lbs from there? The catalogues all have Torque in ft lbs or in lbs
The Torque calc will be Force/wheel radius=10333/0.28m=36903 - am I correct here? Numbers are looking a bit too large for my liking!!

With your help I'm getting somewhere but struggling to get to the bottom line!!

Best regards
Tony

Personaly i dont think it will take much to move it at all. I have moved by hand a 20 ton rail wagon, not far, but it did move with a bit of grunt. Rolling resistance of steel wheels on steel rail is not much. As said its all in the bearings' friction for resistance, really. Looking at the trains running around here, they are at 1hp per ton, or less. Thats with grades. Different scenario i know, but its not much power. Nm is newton meteres, like foot pounds.

1 N-m is 0.73 lb-ft

Your F calculation is force and will be in Newtons, not N-m. This is the force needed to accelerate the rail car. From this you will need to calculate how many N-m you need based on the wheel size. Since the wheel is 280mm diameter, you will need to use 0.14m for the radius to get your torque required at the wheel.

T=Fxr = m x a x r = 10,000kg x 0.083m/s^2 x 0.14m = 116 N-m

While I didn't get out my calculator, I think you have about 6 hp worth of hydraulic flow. For a 5 long ton load I think you will need more power.

Tony

<<I'm working then with a mass of 10000kg (100000N?)>>
- Correct

<<Say accel to 50m/min in 10 seconds therefore 0.0833 m/s^2>>
- 50 m/min ~ 0.8 m/sec in 10 sec, hence ~0.08m/s per sec
Hence Correct


<<therefore F= 100000N x 0.0833 = 8333Nm plus rolling resistance at 2000Nm =10333Nm>>
- OK here I need to clarify units
The unit of Force is N, not Nm (which is a unit of torque; or in other cases it's a unit of work done)
so your calc is correct numerically, but the units of the answer should be N

Your rolling resistance of 2000N (about 200 kg force) seems sufficient, even generous, but for design purposes it might pay to add a factor of 1.5 or 2 because you really don't want a drive like this to be trying hard.

<<How do I get to ft lbs from there? The catalogues all have Torque in ft lbs or in lbs
The Torque calc will be Force/wheel radius*=10333/0.28m=36903 - am I correct here? Numbers are looking a bit too large for my liking!!>>

Torque is Force TIMES wheel radius* (not divided by), which should please you as it means the torque is MUCH lower than you calculated.

I haven't checked the nitty gritty of MBensema's post but have no reason to doubt he has it right in relation to the force required for accelerating the load, but you will of course need to add in the rolling resistance.

The conversion from Nm to ft lbs (NOT ft/lbs, as so many semi-technically literate people mis-state) rounds up to
1 Nm = 0.74 ft lbs, also known as lbf . ft (pounds-force foot)


* As stated above by others, you need to make sure you're using wheel rolling radius, not diameter

gbent said:

While I didn't get out my calculator, I think you have about 6 hp worth of hydraulic flow. For a 5 long ton load I think you will need more power.

Click to expand...

I think (at 10 tonnes) it's more like 9.8 long tons, not 5 ?

Troup said:

I haven't checked the nitty gritty of MBensema's post but have no reason to doubt he has it right in relation to the force required for accelerating the load, but you will of course need to add in the rolling resistance.

Click to expand...

I neglected to put in all my assumptions in my previous post. The torque required to accelerate the load will be much higher then the rolling resistance, this wagon is only going 3 km/h at full speed. Since he only needs to go 15 meters, I assumed he will need to start braking half way there, so the motor will only be driving 7.5 meters or about 8.5 revolutions of the wheels, but he could cut the motor out when he gets up to speed after about 4 meters since the wagon will have enough inertia to keep going until it needs to brake.

I think if he sizes for the acceleration he will be fine.

Once again thanks to all for taking the time to share your knowledge. Everything looks much clearer to me now. MBensema, thanks for your input - your reasoning seems sound, initial force to accelerate the load should logically be more than enough to deal with the rolling resistance once motion has started, however should the rolling resistance not be added to the inertia of the load to start it moving?

Troup, by breaking down my muddled thinking, and clearing up the units, and dealing with it logically you have done me a great favour - thanks.

Gbent and number2 - thanks for your input, if you get a chance I would appreciate your checking our combined "results".

This can be a really good job for me if I get it right first time, if I don't it could cost me. The furnace charging line is the first of three alongside one another, and if i do it right I will almost certainly continue with the following two lines.

Best regards
Tony

I agree with your proposal, Tony, to factor in a generous amount for rolling resistance; it is indeed present from the first moment, and although in a perfect world it will indeed not be appreciable, it only takes a small lump of ore on the track (or any number of unforseeables) for the rolling resistance to shoot up.

I would urge you not to fall into the trap of designing for perfect conditions, especially in dirty circumstances implied by furnaces and bulk ore; nobody's going to be impressed that "it should work perfectly well in theory", and I don't think they'd be keen on employing small boys to sweep the rails.

MBensema: it seems to me that your statement "The torque required to accelerate the load will be much higher then the rolling resistance, this wagon is only going 3 km/h at full speed" is a non-sequitur.

The low top speed means a very low rate of acceleration is required, hence low accelerating force, whereas rolling resistance is not in any sense diminished by the low speed - if anything, it has to be taken more seriously at low speed because there's little inertia available to help the trolley deal with localised retardations.

Perhaps you are thinking of air resistance? Which I agree is obviously not a factor.

Rolling resistance to me would be the friction in the axle bearings as well as resistance of the wheels to the rotation at the track. I did not take into account obstructions on the track. Since this is steel on steel, the wheel resistance should be low and the bearing friction is dependant on load and speed, so should also be very low relative to the acceleration torque.

The torque I calculated is the minimum required to accelerate the load due to it's loaded weight, it is up to Tony how much a safety factor he wants to add in there for other factors. If you feel there is a significant rolling resistance, then add it in. I think it will be low enough that it will not make much of a difference since he said speed is not an issue.

I have been following this thread with great interest and want to say what a pleasure it is to read. I'd like to suggest something about your design.

One, as Troup has pointed out, but in different terms, the hardware and capacity is relatively cheap and the cost of a mis-calculation and real-world factors like grunge and non-greased bearings so high that it will pay to put a HUGE excess capacity factor in the drive. Can't think of a case where I've said "damn, I wish I'd have used a less powerful drive".

That said, if you do make a robust system that is way higher capacity than you think you need, design in a specific and easily repairable or even reset-able failure point where the maintenance guys will thank you for your foresight. We see this, or the lack of it, all the time in our plant. Take belt or chain drives as an example. We didn't like the high maintenance associated with these on material handling screws (augers) and started switching to direct drives. Maintenance went down, but, when they jam up we explode things like gear-reducers instead of just smoking off a belt or busting a chain. We are looking for a solution like a viscous coupling or other clutch-like device to limit or control torque at a specific point. Think about controlling that failure point and maybe designing in a limit so when the trolley smacks into the fork truck, you know what's going to give, and it's not that bad.

I'd say our biggest weakness in designing and building our own equipment, which we do a lot, is designing in the maintainability. How do you take it apart easily, etc. The second version, if you have some operating time before you build it, will be better, and the third better yet. Evolution!

Good luck on this job.

Jeff

I also like the thread...brings back old memories.....

Anyone remember the DEW Line and the radars the US had up there in cold as a .... never mind... Anyway all those were driven very slow to reasonably fast with 22 cubic inch / rev hydraulic motors build by Cincinnati Mill... yep, later changed the Co name to Cincinnati Milacron.

Jeff_M_PA said:

We are looking for a solution like a viscous coupling or other clutch-like device to limit or control torque at a specific point.

Click to expand...

Cincinnati Mill used a torque limiting coupling in the feed gearing of all knee and column and the Vercipower Milling machines... Rather simple, imagine a hub with aprox 24 holes drilled spoke fashion containing springs and topped off with a ball... then outside of this is a ring with 24 slightly smaller holes drilled in it. The outside ring was driven by the inside hub and its springs and balls and if someone tried to run the machines table or saddle into an imoveable object, the springs compressed and made one hell of a racket as the balls sliped out of one hole into another waking up everyone... Only problem with this is that if it was done a lot, it eventually wore out the holes in the outer ring and it became easier to make this SAFETY CLUTCH slip... then some mechanic usually got a bright idea and welded the 2 together in a few places... Wreck it one more time, and then something elsewhere in the machines feed gearing blows up and that really costs some bucks to fix.

MBensema said:

.... bearing friction is dependant on load and speed

Click to expand...

If bearing friction depends on speed, (your implication being, if I understand correctly, that it increases with speed) this is something I am keen to learn more about.

Could you explain in what way, and for what reaons?

This description only applies to rolling element bearings, I don't have the formulas for journal bearings, but would assume it is somewhat similar.

The resistance in the bearing has two components, the friction due to load and friction due to viscosity of the grease or oil. You can read the details at this link from ********* Bearings:

http://www.schaeffler.com/remotemedien/media/_shared_media/library/downloads/wl_81115_4_de_en.pdf

Scroll down to page 17.

Basically, the load friction is due to the load on the bearing, diameter of the bearing and the frictional coefficient of the rolling bearing. This is constant. The "load indenpendant" friction is due to the resistance from the viscosity of the grease and the rotational speed. The key with this one is it is the operational viscosity of the grease, so from slow rotating bearings, the resistance can be higher then faster bearings since the viscosity of the grease decreases as its operating temperature increases.

Not knowing which bearings the wagon is using, we can't know for sure how significant this will be, but it can become a relatively large portion of the bearing friction, especially with high viscosity greases and cool operating temperatures.

MBensema said:

I don't have the formulas for journal bearings, but would assume it is somewhat similar.

Click to expand...

Hmm - I doubt it .... sliding and rolling friction are quite different phenomena, both in theory and in their behaviour 'in the wild'.

MBensema said:

... from slow rotating bearings, the resistance can be higher then faster bearings

Click to expand...

Phew - that's a relief. Your previous posts suggested the opposite, which had me worried.

I agree it's barely relevant to the OP's situation, but in some friction sensitive situations I have designed for, it would be.

I was important for me to know if you had discovered something wrong with "conventional wisdom" theory, even if practical experience did not seem to back it up. Sometimes we only notice what we expect to notice.