OT - Can Load Cells intermittently fail?

sbaum

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Mar 2009
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Ontario
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We have a batching system at a customer's site that was randomly (every couple days) not completing the ingredients.

Last night I was on site and the scale reading suddenly jumped an extra 500kg or so then slowly settled back to it's proper reading. Luckily I was there to see this happen, as it is why the incoming ingredients were stopping prematurely.

Normally this scale is very stable.

My question is...
Does this sound like a load cell issue to you or a possible mechanical interferance?

I don't see anything mechanical hitting the mixer however the load cell is on a bearing linkage setup under the mixer and I'm wondering if the bearings are bad and could be somehow affecting it.
 
To test for mechanical problems step on and off the vessel and make sure the displayed weight returns to the same value. Do this at each corner if possible.

To test a loadcell, bang it with a hammer. Make sure the displayed weight returns to the initial value. You will most likely see the value jump a few hundred KGs when you find the bad one.

Based on your description of it suddenly jumping and slowly returning back, I am betting on a mechanical problem.
 
May need some more details on your setup. Things I can think of off hand are a loose loadcell and noise. IF you are using multiple loadcells then it is possible to get a false reading if one of the loadcells is not completely bolted down or is resting against something (some kind of mechanical binding). The noise can be due to bad grounding. I have seen where VFDs improperly grounded causing noise on the milivolt load cell circuit.

To check for mechanical issues try a corner test with test weights. For a noise test try bringing motors online without changing weight and see if your values change.
 
Yes load cells can intermittently fail.

I see it all the time but as most have already said most of the time it's a mechanical issue that gives you false reading then returns to normal readings.
 
My take:

If the load cells are not exposed to excessive force or shock (which is worse than constant force), then they should not intermittently fail.
As long as they are not exposed to neither excessive force nor shock they should last very long.

But since the measured voltage is very little, the converted signal is very sensitive to bad connections between load cell and converter. I have seen some solder all connections between loadcells, summing boxes and converters in an attempt to fight bad connections.

edit: NEVER HIT A LOADCELL WITH A HAMMER !!!!!
 
I've been building and repairing scales for 20 years. The first thing I do on site when a customer reports intermittent weighing is bang the load cell with a hammer. I have never had any issues. If the loadcell is in poor enough condition to be damaged from a tap, it is already doomed.

Think about a dump truck driving over a truck scale. Surely that is more impact than a tap from a 2lb hammer.

Most of my issues seemed to be from moisture in the load cell. The hammer trick seems to bring any under lying issues to the surface.

To each is own though, this is just the way I do it.
 
Last edited:
sbaum said:
We have a batching system at a customer's site that was randomly (every couple days) not completing the ingredients.

Last night I was on site and the scale reading suddenly jumped an extra 500kg or so then slowly settled back to it's proper reading. Luckily I was there to see this happen, as it is why the incoming ingredients were stopping prematurely.

Normally this scale is very stable.

My question is...
Does this sound like a load cell issue to you or a possible mechanical interferance?

I don't see anything mechanical hitting the mixer however the load cell is on a bearing linkage setup under the mixer and I'm wondering if the bearings are bad and could be somehow affecting it.
Click to expand...

Assuming this is a hopper scale with more than one load cell, could be an intermittent connection in the summation box or at the indicator end. Wiggling wires can determine this.
 
I have seen issues when the power to the transmitter is the same power that supplies an inductive load. Every time the inductive load ( solenoid) dropped out you would get a spike from your transmitter.
 
Thanks to everyone for the feedback.

It is a single load cell and as I previously mentioned, the mixer is weighing properly when tested. However when the mixer is almost full, every now and then it will spike up.

The electrical environment is very clean... no vfds, etc.
The cable shield was not previously grounded, but I've done that now.

I am leaning toward a mechanical issue and am asking the customer to service the bearings linkage the mixer is resting on.
 
Mickey said:
I have seen issues when the power to the transmitter is the same power that supplies an inductive load. Every time the inductive load ( solenoid) dropped out you would get a spike from your transmitter.
Click to expand...

I have also experienced this in the past. In those cases, I usually lost RS485 communications to the transmitter.

I am using a 24vdc Laurels transmitter. There shouldn't be any inductive loads. I have a separate power supply that is just for the transmitter and my devicenet network, which is only a few local nodes.
 
gbeaker said:
I've been building and repairing scales for 20 years. The first thing I do on site when a customer reports intermittent weighing is bang the load cell with a hammer. I have never had any issues. If the loadcell is in poor enough condition to be damaged from a tap, it is already doomed.

Think about a dump truck driving over a truck scale. Surely that is more impact than a tap from a 2lb hammer.

Most of my issues seemed to be from moisture in the load cell. The hammer trick seems to bring any under lying issues to the surface.

To each is own though, this is just the way I do it.
Click to expand...
It depends on the loadcell.
For a truck weigher, I guess there are several loadcells in 10-20 ton range, or not ? For such load cells a hammer may not do any damage.
Loadcells normally tolerate only 50% overload (depends on the loadcell, for some applications we use a special rugged design that tolerate 100% overload).(*)
We have one small loadcell type (500N) where the manufacturer says that it can be damaged if dropped on the floor.

*: Just checked again. The normal loadcells we use (HBM) are rated to 150% of the nominal range (50% overload), and the special type (Eilersen) are rated "up to 1000% overload" !
 
Last edited:
gbeaker said:
I've been building and repairing scales for 20 years. The first thing I do on site when a customer reports intermittent weighing is bang the load cell with a hammer. I have never had any issues. If the loadcell is in poor enough condition to be damaged from a tap, it is already doomed.

Think about a dump truck driving over a truck scale. Surely that is more impact than a tap from a 2lb hammer.

Most of my issues seemed to be from moisture in the load cell. The hammer trick seems to bring any under lying issues to the surface.

To each is own though, this is just the way I do it.
Click to expand...


Heh!

Obviously, there is a fine line between "tapping" and "whacking". ;)

No offense intended on my part.
 

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