Vorecon 1

[rQuestionEngineering]

Vorecon design is convenient for variable speed and power. It is supposed to be highly reliable and durable. But when there is a problem, a vendor is needed to fix it, which takes days or weeks to arrive due to Visa / Work Permit problems. As it is highly engineered, the parts might take time or it could be very expensive.

Compared to VFD and simple bevel gear configuration, what good sense can be justified to spec in HV motor - Vorecon configuration ?

https://voith.com/corp-en/drives-transmissions___variable-speed-drives.html

 

[Compositepro]

The Vorecon contains a torque converter and operates as a variable ratio gear box. It can multiply torque of the motor as speeds are reduced. A VFD cannot increase torque output from the motor as speed is reduced. So high inertia loads are started much easier with a Vorecon than a VFD motor. If you change to a VFD you will need a larger motor.

 

[Windward]

Artemis has been developing an entirely hydraulic variable speed drive. See the illustration and other information on their website:

http://www.artemisip.com/technology/

They claim:

dramatically lower energy losses (typically less than a third of swashplate machines)
dramatically faster response (typically ten times faster)
complete elimination of high-frequency noise

Compositepro, good comment. It looks like this device should be able to produce any torque up to the maximum regardless of speed.

 

[Compositepro]

Torque times rpm is equal to power. Power (or torque input) is limited by the drive motor. So I am not sure what you mean by the statement "It looks like this device should be able to produce any torque up to the maximum regardless of speed." The same can be said of the motor (with external cooling).
You cannot increase torque and speed together.

 

[Windward]

Compositepro, have a look at the information on the internet about hydrostatic drives. For instance:

"Hydrostatic drives are widely recognized as an excellent means of power transmission when variable output speed is required. Typically outperforming mechanical and electrical variable-speed drives and gear-type transmissions, they offer fast response, maintain precise speed under varying loads, and allow infinitely variable speed control from zero to maximum.

"Unlike gear transmissions, hydrostatics have a continuous power curve without peaks and valleys, and they can increase available torque without shifting gears. But despite the superior performance of hydrostatics, a major drawback has been higher cost compared to their mechanical counterparts...

"A basic hydrostatic transmission is an entire hydraulic system. It contains pump, motor, and all required controls in one simple package. Such a system provides all the noted advantages of a conventional hydraulic system -- such as stepless adjustment of speed, torque, and power; plus smooth and controllable acceleration; ability to be stalled without damage; and easy controllability -- with the convenience of single-package procurement installation..."

https://www.machinedesign.com/mechanical-drives/hydrostatic-drives

The main claims for the Artemis device are that it is more efficient than previous hydrostatic drives and that the control is faster and more accurate.

I think you are missing the fact that in a hydrostatic drive, the pump driver can operate at high power, generally meaning high speed, while the motor can operate at the same power but at a lower speed and higher torque. That is the relevant difference here between hydrostatic drive and a VFD or any other device in which the output speed and torque are closely or directly related. In those devices, when the speed of the prime mover goes down, so does its torque and power. The hydrostatic drive removes that relation.

 

[Compositepro]

The fact that I am missing is what point you are trying to make. You are stating the obvious and not contradicting anything I've stated. You make a questionable statement that is probably just a matter of word choice rather than lack of understanding.

"when the speed of the prime mover goes down, so does its torque and power. The hydrostatic drive removes that relation." An electric motor can output full torque at reduced speed. It may require external cooling because the internal fan is less effective at lower speed. I do not think that there is any disagreement.

 

[Windward]

Compositepro, thanks for the correction about the torque of an electric motor. It does not decrease with speed, at least in an induction motor. My recollection of the curve got scrambled over the years.

The purpose of the Vorecon is to allow changes in the speed ratio, within limits, without interrupting the power transmission. But its operation is not like changing to a gearbox with a different ratio. That would change the output torque and speed for given input torque and speed. In the Vorecon, if the motor torque is constant, the output torque is constant regardless of the input and output speeds.

Why is that? The Vorecon torque converter changes only the speed of the sun gear. It cannot change the output torque of the unit. The torque on all elements is determined solely by the torque of the motor acting through the fundamental ratio of the gearbox, that is, the ratio when the sun gear is stationary. If the torque converter forces the sun gear to rotate, it will not change any torques. It will change only the output speed of the unit.

I guess the same is true for a VFD, except that there is no gearbox. The output torque is simply the motor torque. I thought that a hydrostatic drive could change the output torque, but I am not sure about it now.

I suggested the Artemis because it appears to be a major improvement in hydrostatic drives. If bayan1999 could use a Vorecon but is leery of repairs, maybe the Artemis would be a better unit for his application.

 

[Compositepro]

The torque in any drive system is determined by the resistance of the driven load to turning (EgineerTex, inside joke).

Hydrostatic drives are not very energy efficient, and this becomes more important as the power increases. The Vorecon is more efficient because only a fraction of the total power goes through the hydrostatic part. Most of the power goes through the gear train, which is inherently more efficient. The Vorecon is an ingenious device that is rather difficult to understand, just as a torque converter is a very simple device but the principles of its operation are not simple to understand, or explain.

The planetary gearbox in the Vorecon essentially has two power inputs and one output. One of the inputs is the main electric drive motor. Some of the power output goes through a torque converter and is fed back to the second input with with variable torque and speed. This results in higher torque and reduced speed in the output.

The concept is somewhat like how the evener works in a horse-drawn plow using two horses. The side-by-side horses pull on each end of the evener bar, and the plow is attached to various points near the center. If one horse is stronger, the plow attachment point is moved closer to the stronger horse so it has to do more work than the other horse. But the power of both horses is added together.

https://www.google.com/search?client=firefox-b-1-d&q=horse+evener

 

[Windward]

Having grown up next to the largest Amish community in the world, with a father who started his farming career with horse and mule power because the Fordson tractor was still on the drawing boards, I enjoyed your plowing analogy. I have seen an evener and a doubletree, but never used any.

I agree that the Vorecon principle is a little tricky to understand. It has one power input, the motor. That input is split between the torque converter and the input shaft of the gearbox. Whatever torque is acting on the input shaft, the motor torque must be greater, because some of it is going to the torque converter.

Does the Vorecon change torque because it changes speed? No.

The Vorecon does not operate like the gearbox in a car. It seems to, because it can produce different output speeds for a given input speed. But in a car’s gearbox, the output speed and torque are changed by changing the gear ratio.

Not so in the Vorecon. It has a fixed gear ratio. That is the crucial point. The Vorecon has no effect on the torques except by that single, fixed gear ratio, no matter what the torque converter is doing and what speed changes are occurring.

Dig out that little gearbox with in-line shafts. Fix the gear case. Put a torque on the input shaft and exactly counteract it with a torque on the output shaft, which is set by the FIXED gear ratio. Holding those two torques constant, start to turn the gear case. This will change the relative positions of the shafts, because of the shaft registration effect, but it will not affect the torques on the shafts. Turn the gear case as fast you want in either direction or hold it still, those torques do not change.

That is how the Vorecon works.

While conventional hydrostatic drives are inefficient, Artemis claims to have solved that problem and made other important improvements. They must be on to something. Danfoss bought the company and is building plants to manufacture the device.

 

[Compositepro]

You just said that a Vorecon is simply a fixed ratio gearbox. That is just wrong. I suggest you try and figure out why anyone would pay $100,000+ for a Vorecon rather than arguing about how it does not do what it is advertised to do. You will never use or see one, but being able to understand how they work will greatly increase your engineering skills.

You may be hung up on the fact the the planetary gears have a fixed number of teeth and therefore a fixed ratio. That is true. But the whole assembly, with the torque converter with variable vanes, behaves as a variable speed gearbox. A car differential also has a fixed ratio. But if you keep the drive shaft speed constant and brake one wheel the other wheel will double its speed. Instead of a brake, a Vorecon essentially uses a variable torque converter between the drive shaft and one wheel in order to control the speed of the other wheel.

The torque converter also acts as a variable gearbox by multiplying input torque and reducing speed. This higher torque is used to react the higher torque output of the planetary gearbox. You will note that none of gears in the planetary gear box are connected to the fixed casing as is typically the case. Just like in a differential.

 

[Windward]

Samuel Johnson said it best: "I have found you an explanation. I am not obliged to find you an understanding."

 

[spigor]

The torque converter branch of Vorecon takes some torque from the input shaft, changes it (and also the speed) and it's added to the output torque of the gearbox. As the output speed is changed, the torque is also changed to maintain the power minus losses (which are relatively small as most of the power flows through an efficient constant ratio gearbox).

 

[Compositepro]

Yes, spignor, very concisely stated.

 

[Windward]

Also incorrect. The torque converter is not properly named. It should be called the speed variator. Its only function is to change the speed of the sun gear. It does not, and cannot, create or add to any torques in the gearbox. Those torques are created by the rest of the unit and the load. The "torque converter" must handle those torques, but it does not create them or change them.

Spigor, answer me this. If the torque converter "takes some torque and changes it", that means that the output torque is different from the input torque. That's exactly what a gearbox does. But the gearcase of the gearbox must be grounded to resist the torque difference. Otherwise the gearcase would just rotate with the shafts and the gearbox would be useless. Now, where is the ground that you need for the torque converter to do what you say it does? There is none, because the torque converter does not do what you say it does.

Get hold of a small gearbox with in-line input and output shafts and try the experiment I described above. That is the best way to understand how the Vorecon works.

 

[Compositepro]

Windward please review the Vorecon website and research how torque converters work before you say such things in public. Google, Wikipedia, and YouTube are very useful for this.

 

[Windward]

You apparently believe that the Vorecon “torque converter” is the same thing in principle as an automobile torque converter. You must understand the difference if you want to know how the Vorecon works.

An automobile torque converter does increase torque, but only during acceleration. Once final speed is reached, the converter locks up to eliminate losses. The device becomes a rigid coupling. Torque in and torque out are equal. The aid to acceleration is no longer needed because the engine can handle any changes directly.

When the automobile torque converter is increasing torque, it does have the ground I mentioned earlier. That ground is the inertia of the hydraulic fluid. It creates the ground through its resistance to changes in speed and direction, otherwise known as velocity.

During acceleration the automobile torque converter operates just like a gearbox, the glorified kind we call a CVT. The higher-speed-lower-torque input is converted to lower-speed-higher-torque output, although the speed and torque ratios are continuously changing. They decrease as the car’s speed increases. But power into and power out of the torque converter are always equal, except for the small losses in the hydraulic fluid flow and from friction in the bearings and shafts.

The Vorecon “torque converter” is entirely different. It has “adjustable guide vanes [which an automobile torque converter does not have, among the many differences in design and function between these two devices] at the pump wheel which control the fluid flow in the torque converter and determine the speed of the turbine wheel. This allows the speed of the driven machine to be infinitely adjusted.” Recognize that? It is from the Vorecon literature. Have you read it? There is no mention of torque multiplication, because that is not what the Vorecon “torque converter” does. The literature mentions only speed conversion, because that is the sole purpose of the Vorecon “torque converter.” That is why the device is misnamed. It should be called a speed variator.

Better get out that little gearbox as I have suggested, and start trying to figure out how the Vorecon works.

 

[spigor]

@Windward
Do you agree that the power from the input shaft of Vorecon is divided into two paths by a differential gear?
One path is a fixed ratio gearbox path.
The second path is a speed variator path.

What is the torque in the speed variator path?

 

[Windward]

Spigor, as you say, the power from the motor comes to a fork in the road and takes it. Most of it goes to the low-speed shaft of the gearbox driving the planet carrier. The rest of it goes to the hydrodynamic coupling, or speed variator, and then to the high-speed shaft and sun gear. The gearbox combines these two power inputs into the total power output which appears at the gear case, or cage, and then at the output shaft of the unit after passing through the parallel shaft gears, the ratio of which is fixed.

To determine torques we need only the fundamental ratio of the gearbox. By fundamental I mean when we observe the gearbox in isolation, with the cage fixed. In the Vorecon illustration, the sun gear and planet gears look to be about the same diameter. If they are the same, and because the gearbox increases speed, then the fundamental gearbox speed ratio is 1:4, going from input to output. The torque ratio is the inverse, 4:1.

Keeping things simple, for a torque of 4 on the low-speed input shaft of the gearbox, the torque on the sun gear and high-speed shaft is 1. Then the torque on the cage must be the difference, so it is 3. Torque direction is by inspection.

It is critical to understand that the ratios of these torques are not affected by any changes in the speeds of the various elements. The magnitudes of the torques will change as the torque imposed by the load changes, but the ratios of the torques are fixed by the fundamental ratio of the gearbox, which does not change. That is the difference between the Vorecon gearbox, distinguished from the overall unit, and one in which the speed and torque ratios can change, such as a three-speed gearbox in an automobile. Yes, I am an old fogy. The fact that these ratios do change in the overall Vorecon unit causes confusion on this point.

If the prime mover is an induction motor running directly across the line, then the motor speed is fixed. This is fine and good. The purpose of the Vorecon is to eliminate the need for a variable speed prime mover such as a VFD. But the speed of an induction motor would not be exactly fixed. It would change a little as the load changes. It would be fixed in a synchronous motor, but enough quibbling.

The fixed speed of the motor sets the speed of the low-speed shaft of the gearbox driving the planet carrier. The speed variator varies the speed of the high-speed shaft driving the sun gear. The combination of these speeds sets the output speed of the gearbox, which appears at the cage. The output speed of the unit is set by the combined speed at the cage and the final stage in the drive train, the parallel shaft gears, which increases the speed of the cage at a fixed ratio.

During a fixed speed ratio for the Vorecon and a fixed load, the speed variator does not increase the torque on the sun gear nor does it extract any. It only meets the torque that is appearing at the sun gear. Otherwise it would be accelerating or decelerating the unit.

But the Vorecon must be able to accelerate or decelerate. Then the speed variator does increase or extract torque from the sun gear, although it is only partially responsible for the power increase or sink. Once the new output speed is reached and the new load is fixed, the speed variator goes back to only meeting the torque on the sun gear. But even during acceleration or deceleration, the torque ratios in the gearbox do not change.

I suppose there could be applications in which accelerations and decelerations occur almost continuously. In fact Voith designed a unit for those conditions. In principle it is the same as the Vorecon, except that it has a more sophisticated means for controlling the speed variator. See the Voith WinDrive for wind turbines.

Going back to the beginning and looking at the load split in the opposite direction, the load on the motor is the combination of the load on the low-speed input shaft of the gearbox and the load on the speed variator.