Off Topic, need RPM sensor for very slow shaft

Ken Moore · Feb 5, 2021

Hi folks,

I have a hydraulically driven rake in a clarifier that rotates at about .18 rpm.
When there is a process upset, the bed of the clarifier can become quite thick.

When this happens the rake sometimes doesn't turn.
The Clarifier is currently being controlled by a Compact Logix. 1769-L24ER-QBFC1B. I am looking for an off the shelf device that I could install to monitor shaft rotation. I have used a gear tooth and prox in the past, but looking for something pre-fabbed.

Any ideas?

AkaHammer · Feb 5, 2021

What about simple prox switch, as slow as speed is that would work with about any PLC input or could you use a zero speed sensor, I forget the brand name(corny name they had but worked great and very easy install) but we used a lot on conveyors many conveyors in a row critical to know if one drop so no pile ups.

We put magnet on shaft end and the entire sensor attached that way with a strap so it would not rotate, it gave a pulse.

AkaHammer · Feb 5, 2021

here is similar version: http://www.ipcd-inc.com/wp-content/uploads/2016/09/PPI-Zero-Speed-Switch.pdf

Whirligig:https://www.radwell.com/Shop?source...3CQ4Onbe_Tdqd5McGRIXj4o45qKK9LthoCtG4QAvD_BwE

Saffa · Feb 5, 2021

Most of the clarifier drives I've seen have a prox like AkaHammer suggested, which is used to detect the movement of the clarifier arm (not on the gearbox shaft, but on the end of the arm at one point on the radius of the clarifier.

We also used to have a torque switch fitted for the DOL motor ones, i think it was just a 3 phase current monitoring device but it's been a long time since I've looked at one.

More recently, as we now drive most with a VSD, I use the built in high torque monitoring feature of the ATV6xx to raise an alarm.

geniusintraining · Feb 5, 2021

They make (forgot the brand) shaft verifying prox monitors they were not real expensive but you could set the RPMs with a pot to give you a digital input... we had them on our fan shafts that way if the belts broke you could detect it, but I agree just about any prox would work at that speed

lesmar96 · Feb 5, 2021

Saffa said:
More recently, as we now drive most with a VSD, I use the built in high torque monitoring feature of the ATV6xx to raise an alarm.

I wondered about this too. We did this for a customer last year. Any downsides to this method?

Saffa · Feb 5, 2021

lesmar96 said:
I wondered about this too. We did this for a customer last year. Any downsides to this method?

It can be a bit fiddly to set up right, especially if the torque available through the gearbox is high, but the motor runs lightly loaded. I would set it close to the running torque then increase up slightly if there were any nuisance trips. Very hard to see what's going on at the bottom of a deep, sludge blanket-filled clarifier so you always have to hope that nothing starts going wrong early on and you don't end up compensating for a legitimate issue.

thingstodo · Feb 5, 2021

Ken Moore said:
Hi folks,

I have a hydraulically driven rake in a clarifier that rotates at about .18 rpm.
When there is a process upset, the bed of the clarifier can become quite thick.

When this happens the rake sometimes doesn't turn.
The Clarifier is currently being controlled by a Compact Logix. 1769-L24ER-QBFC1B. I am looking for an off the shelf device that I could install to monitor shaft rotation. I have used a gear tooth and prox in the past, but looking for something pre-fabbed.

Any ideas?

Do you have access to an open bearing on the final drive? Without that, it's tough to mount an off-the-shelf unit.

Any existing brackets that can be used to mount it? Size restrictions? What power is available (24VDC, 120 VAC, 250 VAC)?

What sort of interface do you have on your PLC to accept the result - Digital input, active low or hi, analog 4-20mA for speed ... I can't think of any others right now

g.mccormick · Feb 5, 2021

Use a prox and have the rising edge reset a timer. It should get triggered every 5.55min. So if the timer gets to say 12mins (or some value you can live with) you know that something is wrong.

OkiePC · Feb 5, 2021

Many of the clarifiers we come across have an assembly that lets the drive chain pop out of place to prevent breaking the rake. Those usually have some sort of limit switch attached that is supposed to be wired into the motor controller or at least to an alarm lamp. 90% of them are either defeated or out of adjustment. If that switch exists and is set up correctly it should function to give you instant notification of a mechanical drive problem.

Tom Jenkins · Feb 5, 2021

When I was designing rectangular clarifier drives we always included a torque limiting clutch like a Centric ( https://www.bostongear.com/products/clutches/mechanical-torque-limiting-clutches ) The plate would trip a prox or limit switch.

Bit_Bucket_07 · Feb 5, 2021

AkaHammer said:
What about simple prox switch, as slow as speed is that would work with about any PLC input or could you use a zero speed sensor, I forget the brand name(corny name they had but worked great and very easy install) but we used a lot on conveyors many conveyors in a row critical to know if one drop so no pile ups.

We put magnet on shaft end and the entire sensor attached that way with a strap so it would not rotate, it gave a pulse.

I've used those. I think they're called Whirligigs.

Rob... · Feb 5, 2021

A standard encoder... or absolute if the system has power cycles and you need to detect if it's moved since.

Off Topic, need RPM sensor for very slow shaft

Ken Moore

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AkaHammer

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Saffa

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geniusintraining

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thingstodo

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