Leg speed & VFD

[ptonsparky]

Customer with a Leg that is 'backlegging'. All of the product is not being thrown out of the cups at the distributor allowing some of it to fall back down to the bottom where it is picked back up for another ride to the top. We noticed it a few months ago and brought it to the attention of the local manager. Something changed, besides the at least three time turnover in employees, because we do not remember them having the problem before.

Corporate has attempted fixes without much success so they sent the problem out to a company with more experience in the field. Their suggestion is to speed up the leg and see if that helps. Motor is 15 HP running at about 8 amps with the load they now have at a leg speed of 54 RPM. I have a spare used 10 HP VFD in my assortment of stuff. Is there any reason not to speed things up temporarily with it? I may even have a load reactor. Extended operation above 60 RPM will require increased shaft and bearing sizes.

Making sure Corporate realizes they buy any equip damage, of course. Personally I can see all sorts of grief with belts and buckets that have always ran at 54 RPM. I do not know the max speed of the gear reduction.

 

[__dan]

Is there any reason not to speed things up temporarily with it?

You would want to cover in your agreement that the change is for a trial or testing purpose and may have to be changed back. There is not a guarantee it will work.

It is the process of elimination. Making a list of all the possible causes and working though that list to find the actual cause. (I am guessing) other causes may include the material being wetter than normal, moisture, some skirting or guarding may be out of place, the mechanical action at the head of the distributor may have changed, possibly by bad bearings, worn shafts, bolts, bushings.

The cup itself may be stickier than normal and need to be cleaned or possibly repainted.

I would call the manufacturer of the equipment first, suspecting this is a problem they have seen before and can suggest other possible causes for.

 

[Jraef]

To my mind, the idea of speeding it up seems almost counter intuitive. If anything, the bucket is passing over the top so quickly that not all of the grain has dumped into the hopper yet before it moves past the apex, so the remainder falls back down once the bucket is fully upside down. If that is the source of the problem, speeding it up would likely make it worse. It might, on the other hand, mean that by picking it up at the bottom again faster, it appears to solve the net grain movement time issue, but likely at the cost of significantly more energy. Then there is likely a point where if you go too fast, you get 40-50% of it not dumping at the top and falling back down, so you easily run up against the law of diminishing return.

Mind you, I've never seen the insides of the top of a grain leg so I'm not sure just what happens to the grain at the top, but I've run across similar issues in mining applications. We refer to this as the recycle rate, because you expend energy to get it to the top, but although it falls back down for free, you then have to expend energy to get it back to the top again. So even if speeding it up makes a given amount of the product clear the input hopper faster, the fact that you had to lift 20% of it twice means that you expended 20% more energy to do so. Might not be an issue if the time is more important than the energy efficiency, that's a local call.

All that said, you could in fact prove that put by putting a VFD on it, at least temporarily, and play around with the speeds. Once you find the right speed, from an efficiency standpoint just change the sheave ratio on the drive belts (assuming such), but if the conditions vary from job to job, the VFD might be a good permanent solution. Just don't leave that 10HP drive on that 15HP motor forever...

Ideally, and what I was able to do in some mines because the return came back on belts, if you can somehow measure the recycle rate you can feed that back into a PID loop in the drive. Then have your set point for the PID loop at close to zero recycle, so the drive automatically optimizes the feed rate. But measuring falling grain would be problematic I would imagine, so you may need to just eyeball it and set the speed manually.

 

[kwired]

Is the leg being used to handle same type of product it was initially installed for?

I don't know a lot about cups used on these legs, but can see their design effecting which one may work better for different products as well. Also they do wear out which may effect performance.

 

[ptonsparky]

They are moving alfalfa pellets and have at this location since the 50s. The cup design and speed of belt relate to the discharge of the pellets at the top of the belt. Much like the air from a centrifugal fan is my SEWAG. That may not be a good comparison but it is all I can think of. Change the shape of the blade or cup will change the product discharged at a set RPM. Changing the RPM changes rate as well as would changing the cup. There is also a plate at the top of the leg that catches the pellets as it is thrown from the cups. The distance from the cups would also cause grief. Supposedly that distance is good, and corporate says the cups have not been changed.

Something changed in that we noticed the difference in noise level.

 

[Cow]

You've verified the distributor and piping isn't plugging?

A couple weeks ago I had a customer with an elevator feeding a distributor that we noticed was doing the same thing as yours. I climbed up there and verified the distributor and piping were clear. It was then we realized their was frozen chunks of grain right where the piping turned to drop into the bin. It would let some grain through, but it would slowly back the pipe up with grain until the elevator was dumping the grain back to the bottom and on and on. Eventually the elevator would trip though, in our case.

My question to corporate would be, has it ever worked properly?

 

[Dzboyce]

I'm surprised there's that many people on here know what a grain elevator leg is! If this elevator has been in place 50 years, I would be much more suspicious of a mechanical problem in the distributor head. Changing the speed of the elevator belt might solve the symptoms, but not the underlying problem.

That being said, it would be very easy to temporarily wire in your VFD and see what effect varying the speed of the belt has.

 

[ptonsparky]

The leg itself has not been there 60 years. I think we helped with the leg install 10 or 15 yrs ago. The facility has though.

Distributor has been checked and rechecked.

 

[ptonsparky]

Thanks for the comments. Corporate went ahead and bought new sheaves. Don't know how they came to a decision for the speed they wanted.