Breakaway torque 1

[Trommel]

From the outset I am not a fluid engineer.

I am investigating a process which requires a flywheel to be turned, In order to build up that initial momentum to get the flywheel turned a high torque is required and hence there is a very high pressure drop across the motor.
The motors we currently use are simple gear motors which are specced for that breakaway torque but do not for the running torque.

I was looking at variable displacement motors which I think can provide that initial torque and give a much more reduced running pressure.

Does anyone have any experience in dealing with this type of breakaway torque situation?

 

[Gkranz]

Most gear motors will only produce about 30% of their rated running torque. All spec sheets list running torque as you may well know. Fixed piston motors about the same as gear. A radial style motor will produce about 92% of its running torque.
You could use a variable displacement motor, however the circuit would have to have some way to sense the reduction in breakaway torque and adjust. You could just rely on the load sense type control, but I have found them to be too slow to respond to the momentum build up from the flywheel.

Your third statment does not make sense, the breakaway pressure should be the highest, then much lower running pressure. And because you have to oversize the motor to get the breakaway torque, this means at running pressure you will be much lower than you normal would be running. Anyway maybe this will help some.
Gkranz

Westerndynamics.com

 

[EdDanzer]

A variable displacement piston motor will work. All hot saws used for falling trees use them on the saw disk. These disk weigh up to 1000 lbs.

 

[Gkranz]

Any rotational load will see the affect of breakaway and running torque, as do cylinder loads but generally not as drastic. A variable displacement motor could work, however it depends on the type of load. A 1000 lb load will not produce that much flywheel affect as it relates to the breakaway torque. When I see the word flywheel I think of a 10,000 lb to 20,000 lb load, something like a flywheel used in rock crushing, or eccentrics used in vibro machines we design.

This question is somewhat open ended, as there is no information as to what the load is, response time needed, and type of control and of course how is the load is expected to slow or stop. What I use for eccentrics may not work in your application, and a hot saw is something totally different as well.

The classic mistake people make is thinking components are made for specific applications, which is not true. We make components to do specific functions, and the application determines what functions are needed. To get a better answer you need to provide much more information regarding the load and application. And seeing the word (Mining) next to your name means this may be a mining application, which means safety must be a big factor in any changes that are considered.
GKranz


Westerndynamics.com

 

[Trommel]

Gkranz I don't quite follow your statements

"Most gear motors will only produce about 30% of their rated running torque. All spec sheets list running torque as you may well know"

Any of the spec sheets I have looked at (David Brown or Danfoss) I have seen charts describing torque along with power, flow rate and pressure. Again I am not a fluid engineer, so please don't laugh if I say something stupid.

The problem is that my company has always used oversized gear motors to turn a flywheel up until now it has worked well, However recently a new product has been launched with a much greater load invloved. Pressures are excessive and seals and motors are failing.

The motors being used require very high pressures to overcome the breakaway torque. (around 100cc/rev)
I, as the foolish young graduate, was asked to investigate.
What drew me to a variable displacement motor was the means of controlling the pressure.

The breakaway torque required is 290 Nm (measured) and the flywheel needs to rotate at 920 RPM.
I would like to try and keep the pressures to a maximum of 1500 psi (In line with the initial system, hoses etc).

I calculate the required displacement being 190 cc/rev (Massive huh??)

Does anyone know a website that has good data on the capabilities of variable displacement pumps

 

[Trommel]

As well
Running torque has yet to be measured but I has been calculated 200 Nm around 70% of the break away torque. I think some of the managers are intereseted in the cost savings.

What are the main types of applications these motors are used with?

 

[waross]

Have you considered an accumulator? It may not be enough by itself but it will help starting and provide better running surge handling if you need it.
respectfully

 

[EdDanzer]

The point Gkranz was trying to make is that a low speeds and startup gear motors are not good. The leakage around the cheek plates and gear teeth limit starting and running torque. If the devise only runs one direction you could use a low speed high torque motor with and overrunning clutch during startup.
It is important to know the actual application if you need more help.

 

[Gkranz]

Trommel,

quote - Any of the spec sheets I have looked at (David Brown or Danfoss) I have seen charts describing torque along with power, flow rate and pressure. Again I am not a fluid engineer, so please don't laugh if I say something stupid.

Answer - There are two different types of torque in the world, breakawy and running torque. A car placed on flat ground, start to push it, it will require more effeort to get it started that it does to keep it rolling, same with loads.
What you are reading in the books is always running torque, the motor depending on type could ONLY achive 30% of that number during stall or first starting the motor under load.

Quote - What drew me to a variable displacement motor was the means of controlling the pressure.

Answer - Impossible because pumps do not pump pressure only fluid. The load with the displacement of the motor determines the pressure. The only way you get to control the pressure is by picking the right motor displacement, the load is fixed.

Quote - I, as the foolish young graduate.

Answer - No such thing, just young ones and old ones, some like me very old. Also, a 190cc/rev motor is not all that massive, double that or triple it then get back to me when it really gets big. Now think outside the box, try thinking of two motors?

Quote - I think some of the managers are intereseted in the cost savings.

Answer - First lesson to learn, cannot increase the load and do more work with cost savings, pass that on to the managers. I might as well break you in right from the start, nothing is free, more work, more money to do it.
Gkranz

Westerndynamics.com

 

[Gkranz]

Waross, (electrical)
Quote - I think some of the managers are intereseted in the cost savings.

Answer - Stop that? An accumulator will not work, has nothing to do with load pressure, only provides additional fluid, this guy is confused enough. I don't mess with your electrons, do not mess with my oily bugs, ha, ha.
Gkranz

Westerndynamics.com

 

[Gkranz]

Waross, Electrical,
Sorry wrong quote.

Waross quote - Have you considered an accumulator? It may not be enough by itself but it will help starting and provide better running surge handling if you need it

My answer - Stop that? An accumulator will not work, has nothing to do with load pressure, only provides additional fluid, this guy is confused enough. I don't mess with your electrons, do not mess with my oily bugs, ha, ha.
Now I got it right, sorry about that.
Gkranz

Westerndynamics.com

 

[waross]

Gkranz
You're right, I stand corrected. Sorry.
My thought was not for more pressure but less flow through the pressure relief valve, with the energy being returned near the end of the accelleration cycle. I accept your comments.
respectfully

 

[Trommel]

Guys
Your help is incalculable.

Ok first off the cost savings involved are generally using the current system, which is both inadequate and expensive when you take into consideration things like blown seals etc. As well no-one really wants to change the pump and prime mover set up.

When I talk about controlling the pressure what I mean is avoiding having to cause panic by respecing all the hoses, new suppliers etc.

How do most applications overcome this difference in breakaway and running torque?

In what applications are variable displacement motors used ??
Thanks guys

 

[Trommel]

Also
Is a load sensing compensator appropriate in this situation.

 

[Gkranz]

Trommel,

Look guy, you are not getting the message or you do not want to hear it, not sure which?
Your quote set the guidelines - recently a new product has been launched with a much greater load involved.

Breakaway and running torque is basic physics we all have to deal with, I think or you should understand the difference by our posts. Hydraulic systems are designed to do a specific application with specific requirements, and some designers provide a little added fudge factor should the application change a little or they were given some bad data requirements. Other than that the system is designed to do that specific job and no more, layman’s terms, just big enough to get the job done.

Here is an example: You have a little vehicle that tows around small two seat aircraft from hanger to hanger. It has worked great for many years and management is happy. Now I come fly in with a 747 aircraft twenty times bigger and heaver and want my aircraft towed to the hanger. Now we do not want to throw management into a panic, because they want more money for parking, towing and etc. Would you say that this is a new product that has been launched with a much greater load involved?

Designers and engineers do not panic; they are a problem solver, which is why we have managers, to whine, cry and panic, that’s their job. You had a machine designed to do a specific job, now it has been changed to do a different job (much greater), it is a new problem not an old problem. Go back to the drawing board and design for the new job, because there is no silver bullet that will fix this old problem.

Some short answers for you – “How do most applications overcome this difference in breakaway and running torque”? Answer – They design for it.

”In what applications are variable displacement motors used”? Answer – Not enough room in this post for 5-6 pages of applications I have used variable displacement pumps.

“Is a load sensing compensator appropriate in this situation”? Answer – NO.

It is interesting to note you say it would take a 190cc/rev motor to do the job, get your check book out it is going to cost you dearly. For that kind of money I could replace all the components, hoses and design a system to get the job done, and still have beer money left over. The other thing is this, “much greater load”, is this like fishing where I caught the big one, was it big or really big? Depends on whom you are talking with, and what they consider big.

OK, here is the bottom line:

First, you have to design a system to turn the NEW load.
Second, do it safely and smart, in other words do not use anything in the circuit that is not required.
Third, then and only then look at what you have existing to work with, if you can use it, do so, otherwise replace it or upgrade it.

So my young foolish graduate, take a pencil, three pieces of paper and produce three options to solve the NEW problem, if you can save some money great, if not so be it, but your primary job is to solve the problem. Managers are going to whine and panic no matter what you do, not your problem, the load is your problem. Welcome to the real world my young friend, I know you can do this if you stop looking for the silver bullet and fast fix. You are going to be a good problem solver or be promoted up to management, time will tell.

Best Regards,
GKranz


Westerndynamics.com

 

[EdDanzer]

Trommel,

Why won’t you provide more information? In order to assist further you must provide the disk diameter, weight, running speed, allowable starting time to reach running speed, and any other loads that will increase starting torque. The working pressure and flow of the current pump is also required.
Guessing at ways to solve a problem will never provide the best most economical solution.
If you don’t know the information requested don’t expect a good solution.

 

[senzerz]

Can you guys tell me another non-Fluid power professional how the situation would be helped if you could measure the torque demand of the flywheel as it was being spun up?

 

[Gkranz]

senzerz,

This is an easy one and we do it all the time if we really want to know the torque.

Take a torque wrench and apply it to the point where the hydraulic motor will drive the flywheel. We are assuming the the flywheel has the max load already attached to the flywheel. Pull the torque wrench, you will see TWO readings. The first is a higher reading which is breakaway torque, and then after just a few degrees of movement the reading will drop to a lower reading, which is running torque. One torque to set the mass in motion, and one to keep it in motion. You need to do this two to three times to make sure your readings are repeatable, then we add a little (about 10%) and we have a very good idea of what torque we really need for the application.

As the flywheel turns faster and faster the second reading generally lowers even more, but we use the first and second readings as our starting point in design work. A word of caution, whatever the flywheel is driving MUST have its max load, otherwise this is a waste of time. We do have sensors that will do the same thing should the torque be really large and a simple torque wrench will not do or takes too much to pull it through.


Westerndynamics.com

 

[senzerz]

What I am trying to understand is if you could measure torque continuously at a reasonable cost would there be value to a hydraulic control system. For instance could you gain greater efficiency or use a smaller pump? If not in this application is there a usefulness between the prime move and the pump?

 

[Gkranz]

senzerz,

Not sure were you are going with this, but I can monitor the hydraulic pressure of a pump over a load range, and with known pump rpm can convert back to HP or torque required over this range. We do it with pressure transducers over time. This tells us how much actual HP or torque is being used during operation of the load.

This is all well and good if the machine is already built, however when the paper is blank and we are designing a new machine this must be all figured out ahead of time. I have seen machines that only use a fraction of the actual HP or torque input during operation. These happens when people hold up their thumb and guess at what is really needed, thus pay more for HP than they actually use.

Good designers figure out exactly what is required before the design process and use about 85% of input during operation and hold back about 15% in reserve.

To take it a step further, I can monitor or calculate the actual work being done, and compare this to the hydraulic pressure and tell you how much is work and how much is wasted energy going to heat because of trying to control flow or pressure within the system.

Very common to find hydraulic systems only 40-60% efficient, but as engines and energy demands go up in cost, the demand for better designs and designers will increase. Right now about 96% of so called fluid power designers even look or take this approach, they are just tickled pink to get the system to work at all. And customers do not know nor do they care about efficiency, until the costs go sky high, then we get their attention. Look at the cost of gas, have gotten your attention yet?

Good question.


Westerndynamics.com