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Assembly line manual lifting weight limit

Assembly line manual lifting weight limit

Assembly line manual lifting weight limit

Hi everyone.

I have an assembly line where workers are manually assembling small(approximately the size of a quarter) products.  Currently we have fixtures which hold a number of the components as they are being processed and are passed along the assembly line between workers by gravity-fed roller conveyors.  The layout is such that each worker lifts a fixture off of the "In" conveyor, performs their operations, and then places the fixture onto the "Out" conveyor.

The issue that I'm having is that we've developed a "new and improved" fixture which does a number of things better than the previous version, but is heavier (85g vs. 25g.)  We are hearing complaints about the additional weight from the line workers and the line supervisors are worried about workers compensation claims. I am of the opinion that the fixtures are not "too" heavy, but rather just different and our assemblers are simply resistant to the change. I understand that ergonomics are a very important part of a manual assembly line, but it must be balanced with functionality.  My struggle is in determining what an objective weight limit may be so that we can either show that the new design is under this or I can have a reasonable goal for re-designing the fixture.

I've found the NIOSH lifting equation which defines acceptable weight limits for lifting operations based on frequency, duration, distance of lift, height of lift, etc... (the new fixtures are 10X lighter that this calculated limit), but this only applies to lifting while standing. Our assemblers are seated while lifting the fixtures from the conveyors and while performing their operations. Does anyone know of a similar guide for seated lifting or have suggestions for another approach to determining a weight limit?

RE: Assembly line manual lifting weight limit

Thanks Kenat.  It sounds like you're referring to the NIOSH equation that I mentioned.  It is exactly what I need, except that it is limited to lifting while standing. I'm hoping that something similar exists for lifting while sitting - or at least some other guideline or method of calculating an acceptable limit.

RE: Assembly line manual lifting weight limit

I'm not sure, but I wonder if those calculations contentrate on what is safe more than what is comfortable, maybe that's the difference you're seeing.

We spent ages discussing how heavy an access cover could be on a piece of our equipment that would get opened maybe a few times an hour at most.  We were debating if it should be less than 5lbs, up to 10 etc.

Did the equation and it said 25lbs or so was fine, which we knew was well over the top and our users would have complained.

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RE: Assembly line manual lifting weight limit

Eighty-five GRAMS?  

Since that's within the limit for a standing lift, and you have no data for a sitting lift, the solution is obvious:

Take away their chairs.


Mike Halloran
Pembroke Pines, FL, USA

RE: Assembly line manual lifting weight limit

Mike, I had the same thought but assumed that each widget had a fixture, but re-reading the OP now I'm not so sure.

There may be another Donkey or two on eng-tips.

As to the OP's problem.  I eventually made a cardboard/paper mache mock up, ballasted it with some arm weights the Mrs wasn't using (boy did I get in trouble when she tried to do her once yearly exercise kick and her weights were at my office 200 miles away) and got everyone that mattered to play with it and see what they thought.

So, as long as you're in the 'safe' working load beyond that it may mean just keeping people happy.

Maybe ballast up the old fixtures and see at what point folks complain.  That's not to say you necessarily pander to them but it may be another data point.

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RE: Assembly line manual lifting weight limit

I'm going to put on my lean hat now.

Why are you picking up the fixture? Does it add value to the part?
Take a look at the non-value add and the value add motions which the operators are doing and reduce the non-value add motions.
Arrange the inbound conveyors to have the minimum of motion such as a short slide into the work station and a short slide onto the outbound conveyor. It will mean re-arrangement of the work area but will pay off many times over.
The easiest way to see the benefits is to video the current process. Analyze the video. Change the process. Video the new process and re-analyze.

Now I will get off my soap box.


RE: Assembly line manual lifting weight limit

While I don't have a specific answer to the question I'll add a link to a resource I've found helpful.  It contains Human Factors related documentation you can read from many, many sources.


Just scroll down a bit and you will see a long list, with descriptions, in the center of the page.  Perhaps one of these documents has already studied the question you are asking.

I'll also echo what BillPSU mentioned and add the following.  

Why are you lifting a tooling fixture, regardless of its weight?  When you move it, especially by lifting are you not increasing the risk that it gets dropped/damaged/etc?  If it gets damaged will the process worker be reliable enough to tell someone about it so it can be checked or just dust it off and keep using it?

Maybe in your application you've accounted for these risks or maybe the fixture isn't that critical or is resistant to damage already.  I just am always leery about the movement of tooling unless *really* necessary or there are plenty of replacement fixtures available.


RE: Assembly line manual lifting weight limit

Surely 85 grams must be incorrect.

RE: Assembly line manual lifting weight limit

I'm with McGyver and Mike Halloran.  You did mean 85 grams?!?  I'm flabbergasted.  How do these people even get out of bed in the morning? Do they have someone lift their beer for them after an "exhausting days work"?

Billpsu has touched on a good point, though.  In addition, what automation solutions might be available to you?

RE: Assembly line manual lifting weight limit

Make sure they take there breaks, and maybe then some. I'd also suggest they get up and move around in between breaks. production might go down, but Workers Comp premiums might be cut down.  

RE: Assembly line manual lifting weight limit

Get your workers in shape by loosing weight and excercise. Too many of these assembly line workers (males and females) have big guts and are soft which are the main reasons for back, shoulder and arm injuries.

Injuries with 85g--hummmm!!!

RE: Assembly line manual lifting weight limit

Pre-note:  This is what I learned/remember from a Biomechanics class in college.

In this case, the NIOSH equation is really calculating what is safe as far as compressive forces on the lower back.  Every lifting guide out there will tell you that you lower yourself to the item to be lifted, straighten your back and lift with your legs.

If you are seated however, you cannot use your legs at all.  Everything comes from the lower back.  If I'm picturing this right and the person would be seated in a chair and lifting something in front of the chair on the floor, there would be no acceptable weight limit.  Doing that without lifting anything is not ergonomically acceptable if repeated more than a few times a day.

Take your typical worker, lets say 180 lbs.  Lets say that 40% of his body weight is above waist level (72 lbs), and that the center of gravity for that section is located in the center of his chest, lets say 14 inches from his waist.

Then the body becomes a lever around the fulcrum of the lower back.  On one side we have 72 lbs that is approx. 14 inches from the fulcrum, and on the other side we have a force that is applied  1 inch from the fulcrum (or whatever you approximate the distance to be from the lower back bone to the muscle).  This requires a force of 1366 Newtons to hold.  Within human capacity, yes, but very stressful over time.  When you're standing, the weight of your upper body is centered over your feet, and your back muscles only have to tug very slightly to keep it there (forces are vectors and vectors are independent).  

Automated stands that they can roll the fixture on to, press a button and have it raised would be the best.

85g?  That number is correct?  That is less than my mouse weighs.  If you can arrange the stations somehow so that the operator does not have to move his upper body to get the 85g fixture, he can just rotate at the shoulder to get at it, then I wouldn't see a problem as there would be no strain on the lower back

RE: Assembly line manual lifting weight limit

85 grams thats weight of a small hamburger for pete sake--not even a quarter pounder!!!what do you have babies working in the assembly line?  May be you should feed them hamburgers to get them in shape if you have warm up periods prior to production time.

RE: Assembly line manual lifting weight limit

I too am flabbergasted at the 85 grams.  I work at a manufacturing facility who builds large trucks and our facilities are from the stone ages.  We have line workers bench pressing prop shafts into place.  Not saying that's the right thing to do by any means, but I feel like your people need to sack up.  

My company put me through an ergonomics class and I remember the NIOSH equation.  I also feel like there were a few other similar ones, but none quite as detailed as the NIOSH.  If its the best you have, I would definitely take some samples of some different people and calculate the NIOSH with both the old way and the new way.  Document and present your case to management and have the supervisors push the workers through the learning curve.  

Also, why was the change made?  Did it improve your efficiency?  Speed up your process?  If management sees this little report, putting a cost savings on the new way will make it very easy to sell your case.

RE: Assembly line manual lifting weight limit

Workers will complain and there may be some that will claim to be hurt after changes. You are not asking anything unreasonable and it is up to the line supervisor to work with his people to sell them on the change. You will be surprised how much a good supervisor can do to prevent labor employees from faking injuries. I've been a supervisor of labor employees in the past and by praising my employees, treating them good, and caring about them will really made a difference when I had to make changes. Most laborers will don't want to screw their supervisors by faking injuries or missing work if they are treated good.  

RE: Assembly line manual lifting weight limit

Involve the line leaders and key assembly people when making the change.  Let them see the benefit to the process of the larger fixture and then let them decide when it is too heavy.  With their buyin, you will immediately sell the line staff.

And go lean for a second and ask why you are picking it up to begin with.....


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