Application of the Month
Synchronizing Speed Mechanically
September, 2015 - View all Application Examples
The tried and true method of synchronizing the speed of multiple stations by linking them mechanically remains a popular option in machine design. This is still seen in printing presses, conveyors, roll forming and packaging lines, just to name a few examples.
A customer of ours recently requested some assistance in designing a conveyor and feed system for a large pipe cutting operation. The long, heavy pipe was to be loaded on a set of rollers that would then move the pipe into a cutting house where it would be sawed into various lengths.
The rollers were to be spaced several feet apart and required consistent speed to smoothly move the pipe along. There was also an acceleration/deceleration element to the profile, as motion would only commence after the pipe was loaded and stopped when the pipe was in position for cutting.
Now this is where the motion control guys want to put a servo motor on each station and, with drives and master/slave controls, manipulate and match the motor speeds driving the individual rollers. But this is overkill and more expensive in a lot of cases. And trouble shooting in the remote area this application was destined for could be problematic.
Then there are the speed control guys who would drive multiple gearmotors through a single inverter to try to speed match the rollers. This could work where there is a closed loop speed feedback system, but with open loop the gearmotors would likely operate at slightly different speeds. Again, this is probably a more expensive option with all the extra motors and feedback controls needed for each gearmotor.
No, in this case a mechanically linked line shaft and gearbox system would result in the simplest drive system with reliable speed matching.
So, what are the issues when devising this system? The first is calculating output torque and speed at each station. The second is to determine how many stations the main power source has to drive and where it will be positioned in the driveline. The third is determining the amount of through torque from station to station to pick line shafts and compare gearbox shaft strength.
Lets take as an example a simple eight station system. We identified, through roller diameters, pipe weights, coefficients of friction and linear speed, that each station requires 72 Nm of torque at 100 RPM. That’s approximately 1 horsepower. So, for 8 stations we need 8 horsepower and will likely use a 10 HP motor operating at a nominal speed of around 1750 RPM.
How do we get from 1750 to 100 RPM? Gearboxes! We can put all the ratio in the main drive box and use 1:1 miter boxes to turn the corners or use a lower ratio in the main drive and add some ratio to the bevel boxes. Which is best? It really depends on how many boxes are in the line.
Inherently it’s usually best to put the biggest ratio in the box that drives the individual load, for two reasons. The first is a gearbox’s output capacity is only marginally affected by the ratio, so make it do the work. Secondly, this allows the components on the input side to be smaller because of the lower input torques transmitted.
Unfortunately, spiral bevel gearboxes don’t offer many ratio options from the through shaft on out the pinion. A 2:1 ratio is about the biggest. Unfortunately, that reduces the input shaft diameter. Why is that an issue? It results in reduced through torque capacity.
If we put the main drive gearmotor in the middle of the 8 units to split the load evenly, the first bevel box’s through shaft has to handle the torque of four gearboxes. That can be a problem if the shaft isn’t large enough. In this case, it was a problem. With the output torque of 72 Nm per box, the input requirement was 36 Nm per box, times 4 boxes, or 144 Nm. The 16 mm shaft in the 2:1 box we considered couldn’t handle that.
By going to a 1.5:1 ratio, we could incorporate a 22 mm shaft, which was strong enough even though the input torque to drive the four boxes increased to 192 Nm. Of course, the input torque required of each box down the line decreased as fewer boxes were driven. But the first box had to work. By manipulating the ratios, we were able to select our 01-III-1:1.5 Tandler spiral bevel gearbox (driven backwards to get a 1.5:1 speed reduction), which was the most cost effective solution to handle both the input and output torque requirements.
The second component required was the main drive gearmotor. We wanted a right angle design with outputs left and right to drive both halves of the system with equal loading. With a 1.5:1 ratio in the bevel boxes, we needed about an 11.5:1 ratio in the gearmotor.
We had two excellent options here. We could select a helical worm gearmotor, which offers good running efficiency at a lower price or our helical bevel gearmotor with slightly better running efficiency at a little higher price.
The kicker here was the constant starting and stopping the application required. While running efficiency was not that different between the designs, starting efficiency was. So the helical bevel design, with the better starting efficiency, was deemed the better long-term value considering initial and operating costs. The final selection was our Watt Drive K70A with integral 10 HP inverter duty motor, also designed by WATT Drive.
We were also able to suggest our EZ2 series of torque tubes. Although often more expensive than a steel rod, a torque tube has an advantage in torsional rigidity, especially in longer lengths. This reduces the torsional wind up experienced driving multiple boxes over long distances and allows higher speeds because of the larger material cross section. To handle the 192 Nm generated out each side of the main drive, we selected the R+W EZ2-300 line shaft in lengths of 6+ feet, custom fit for the design.
There are several important things to learn from this type of application. Mechanically linked drive systems can provide cost effective and very accurate speed matching regardless of load variations. By manipulating ratios between the main drive box and the individual station boxes, optimal sizes and potential cost reductions can be realized. When multiple boxes are being driven, overall torque and span between boxes needs to be considered to select the right line shafts. And finally, when you have applications like this, DieQua is really the best to work with because we’ll help analyze the options, and we have the system components to make it work.
Although we have all the position control and speed control gearboxes you could possibly want, we’ve been designing lineshaft driven systems for over 25 years. They still make sense in a lot of instances. Let us help you determine if your application is one of them.
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