Waste-water system clarifier torque limiter testing, help? Anyone in waste-water?

[hockeyoligist2]

I work for a waste-water company. Recently one of our clarifiers hung up on the "beach" and was damaged very badly. The torque limiters didn't stop it. The torque limiters are electrical and mechanical. Different brands and models at different locations. 36 in total.

I got a work order to check them all out and make sure that they work properly.

I can check the limit switches to make sure they are working by making them manually but have no idea of how to test them under load except chaining them off and turning them on. That could be very destructive! High gear ratio, long arms without much support. A millisecond with it chained off could cause a lot of damage!

If anyone is familiar with testing them I would really be grateful for any help!

 

[gar]

111220-2113 EST

I am not familiar with the devices you are describing. I have no idea what they look like or the torque and power levels with which you are working.

Generally a mechanical torque limiter will be based on a spring loaded slip clutch. This is coefficient of friction controlled and probably won't provide precise limiting. But it is a good fast acting device and can provide good protection if properly designed. A limited slip automotive differential is such a device. Millions are produced. When you go around a corner on dry pavement you may feel or hear it slip. Ford Escapes and derivatives of the drive train in Ford cars use use a modulated electromechanical slip clutch to control torque to the rear wheels.

Another type of mechanical torque limiter uses some mechanical part that is designed to fail, usual shear, and therefore called a shear pin, that breaks under excessive torque in a location that does not cause severe damage to other components. This is equivalent to an electrical fuse.

These above devices can operate in milliseconds and if located near the final load can disconnect a high inertia high energy source from the final load very quickly.

Electrically, unless done thru some sort of electromechanical clutch, are likely to control the current, thus torque, to the driving element of the system. Driving probably means motor. Here it could be controlled via limit switches that you seem to have implied, or by modulating the current to the motor.

Any of these methods are going to require you to understand the theory behind their operation.

How you determine whether the device will work will be dependent upon on its design.

A shear pin would not be directly tested. That would causes its failure, but you might check the material and its hardness.

A slip clutch likely would be tested to determine its slip point by a gradual increase in torque. This might be as simple as using a large torque wrench. 300 #-FT is about the largest normal torque wrench without a multiplier. I am aware of 3 times multipliers.

Other indirect means might be used as needed.

Systems designed with strain-gage torque transducers must have mechanical slip protection or transducers that can never exceed the gage stress limit under any load condition. This means operating way down range of the full scale rating of the transducer. Same for pressure transducers.

.

 

[buddhakii]

My only suggestion would be to check with the manufacturer of the clarifier and see what their recomendations are.

 

[hockeyoligist2]

My only suggestion would be to check with the manufacturer of the clarifier and see what their recomendations are.

I have called two of them. They said someone would call me back, haven't gotten a call yet, been 3 days.

 

[hockeyoligist2]

Gar, Most of them have a spring loaded worm gear that shifts as the load increases. The worm gear pushes a shaft with cams against two micro-switches, the first one sets off an alarm the second shuts down the motor.

After a little mechanic work this morning on the one that had the damage I found that the shear pin had been replaced with a solid pin. I replaced it with the proper pin. However, the electrical side should have stopped it also. The worm gear probably didn't shift. I tore it apart and found a lot of corrosion on the shaft that makes the limits but I think the worm gear should have overridden the corrosion. The shaft was not stuck.

Everything looked good on the worm gear and spring, no corrosion, but they are submersed in oil. I took a crow bar and I could move it and make the limits, but I have no idea how much pressure I was applying. That is the catch, I have no way of measuring the inch pounds needed to make the limits at a safe level.

The newer ones have a clutch system, manufactured by Brunel Corp instead of a shear pin. I also called them this morning, no response yet.

Since the arm moves really slow, I think some sort of hydraulic restraining device to measure inch pounds are probably used to test them. I'm hoping, If someone gives me a call back, that they have someone in my area that can do a test. Then I have to talk the boss in to spending the money to get them tested, He's tight, so that might not happen!

 

[Jraef]

If it were me, I would add an "electronic shear pin" device that is looking at motor shaft kW (current alone changes with voltage, so it's not as reliable). Many of the major mfrs have them available now, some of them can take the place of an Overload Relay in the motor controller. I would put them in in conjunction (conJUNKtion) with the mechanical junk, I trust electrons to behave reliably more than metals. if nothing else, it gives you a redundant check on torque. I can spout off a few suggestions if you don't know what I'm referring to but if you describe what the motor controllers are first it would be helpful.

 

[gar]

111221-1855 EST

hockeyoligist2:

The technique of using a spring loaded worm shaft and a limit switch can be a reasonably accurate fixed point torque detector for over torque detection. Pinion preload adjustment machines on which I had electronic equipment used this technique, and this was the only overload detection method until we put an electronic torque transducer on the nut shaft.

There were and are a large number of these pinion preload adjustment machines that have worked successfully.

Suppose you have a 31.8" pitch diameter worm gear with 100 teeth. Then the effective radius is 15.9", or 1.325 feet. An axial force on the worm of 755 # corresponds to a torque of 1000 #-ft from the worm gear. If a spring on the end of the worm shaft applies 755 # of force, then the worm shaft will remain against its seat thrust bearing until the torque load exceeds 1000 #-ft. Above this torque level the worm shaft would axially move and operate a limit switch. That switch would drop power to the motor. Probably by a circuit that releases the holding circuit on an electrically maintained contactor.

You can check this torque limiting system and determine its trigger point by applying a known axial force to the worm shaft. You can probably use a suitable lever arm (a moderately long rod) and a fish scale. A 10 to 1 multiplier (10 ft and 1 ft) and a 100 # scale would work for the above example. Another way is to calibrate an Enerpac hydraulic cylinder for force vs pressure and use it to actuate the worm shaft. Hydraulic to pressure is not too linear so it needs calibration or use a loadcell with it to measure force.

I have used a Load Controls, Sturbridge, MA PH 3A 460 V 10 A 0-10 V transducer to deduce nut torque on a preload machine that did not have a nut torque transducer. It was useful and had fast response. Basically 3 * 120 Hz is the ripple frequency.

http://www.loadcontrols.com/downloads/ph3a_1000datasheet.pdf

.

 

[hockeyoligist2]

Thanks Gar, some of them already have Load Controls. They don't want to add anything to the ones that don't. They just want me to test them to make sure they work.

One of the manufacturers called me back yesterday and said that they can send out a tech guy to test them. $120 an hour plus travel expenses ! Boss says no way!

The fish scale thing sounds interesting, I'll run it by the boss. He will probably go for it since it is a cheap way out. If I can do the math correctly!

 

[gar]

111228-0935 EST

hockeyoligist2:

Can you post a picture of the mechanism that is used for overload torque detection that you need to test?

If you apply a torque input to the motor shaft, then the output mechanism has to be restrained from moving to do a test. Also realize that most worm reducers have a mechanical efficiency of 50% or less.

If you use axial force for the test method, then the output does not need to be restrained.

With what torque levels are you working? At the output of the worm gear is it 10,000; 1,000; or 100 #-ft? What is the reduction ratio from motor to output shaft? Is this all in the worm reducer, or is there also some belt reduction?

.

 

[weressl]

Yep they are highly corrosion resistant.:lol:

You can also 'test' the trip levels by adding some sort of drag device at the end of the arm.

 

[Open Neutral]

111221-1855 EST
You can check this torque limiting system and determine its trigger point by applying a known axial force to the worm shaft. You can probably use a suitable lever arm (a moderately long rod) and a fish scale.
.

How about a torque wrench?

 

[gar]

111231-2349 EST

Open Neutral:

A torque wrench of the correct range is good if you can connect it to the worm shaft and lock the output shaft of the worm gear. Also you need to have the torque wrench. If it is a 300 #-ft unit and you do not have one, then you may not want the expense of purchasing one.

.

 

[Open Neutral]

111231-2349 EST

Open Neutral:

A torque wrench of the correct range is good if you can connect it to the worm shaft and lock the output shaft of the worm gear. Also you need to have the torque wrench. If it is a 300 #-ft unit and you do not have one, then you may not want the expense of purchasing one.

.

How about this and that...or perhaps?

Will that do the job?

 

[Open Neutral]

We use one of these on another project to measure [vs. set] torque. With a long enough leverarm, it may work here.

 

[masterinbama]

If it were me, I would add an "electronic shear pin" device that is looking at motor shaft kW (current alone changes with voltage, so it's not as reliable). Many of the major mfrs have them available now, some of them can take the place of an Overload Relay in the motor controller. I would put them in in conjunction (conJUNKtion) with the mechanical junk, I trust electrons to behave reliably more than metals. if nothing else, it gives you a redundant check on torque. I can spout off a few suggestions if you don't know what I'm referring to but if you describe what the motor controllers are first it would be helpful.

Jraef is giving you a good suggestion. I'll let him correct me on this but I believe if you are using standard starters that GE has something in their multilin line of MPR's that will do what he suggests. The torque limiters in most clarifiers truck along for years and years without being tripped. They corrode internally and the external corrosion adds mechanical drag to them.

If you have ever been on site when the grout bed is swept in, you know that the difference between a slight over torque and arm destruction is not very far apart.

 

[Joethemechanic]

Hi, my name is Joe. I was a rep for DBS clarifier drives for years. I worked on them all though. ENVIROREX , Dorr-Oliver, whatever.

So your scum skimmer hung up on the beach and did some damage? This didn't hang one of the rake arms and collapse your torque cage did it?

Most of the over-torque alarms and high cut-outs are either in-op or adjusted wrong so what you have going on is a common problem. How can I help?