Reducing the mass of a mechanical design 4

[dejan95]

Hello everybody.

I'm a fresh mechanical engineer, only been working for a good two months in mechanical design.

For my new project I got instructions to reduce mass of a cover for guide pulley on the belt unwinder. Every time the belt needs to be replaced, the worker has to take of the cover (12kg) which is not an easy task. I would like to reduce the weight of the cover to at least 6 kg.

Additionally I would love to make easier/quicker locking system.

Things I considered regarding weight:
- making parts from aluminium to reduce some weight;
- reducing the diameter from 1070 mm to 950 mm (it would still serve the same propose);
- reducing the thickness from 3mm to 2 mm.

I have attached some additional pictures for better understanding.

I would really appreciate some help from you more experienced engineers.

 

[SnTMan]

Make the main plate from perforated metal or drill it?

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[hendersdc]

Start by listing out in detail the requirements of the cover. This isn't just an exercise, if you want to change anything about an existing design you need to understand all the constraints around it to make sure you don't solve one problem but create another.

- What does it need to do?
- What does it need to interface with? (consider size and tolerances of all the various parts it mates with)
- What environment does it operate in? (chemical and temperature wise)
- What are the life requirements? (how many cycles, what are the wear points)
- What forces will be acting on it?
- Are there any safety issues?
- etc

Is the person that designed the cover still at your company? If so you can ask them why it was made so heavy in the first place. There could be a reason or it could just be because it was a quick design and weight wasn't a concern at that point.

 

[JohnRBaker]

I agree with hendersdc, start by looking at why it was designed the way it was. We used to call this 'value engineering' and it worked best when redesigning something that had been around for awhile, but the first thing you need to do is take the old item apart, literally or virtually, whichever is practical, and determine the use, or 'value', of each component. Also look for opportunities to either eliminate something or combine it with something else. And while you're at it, look at the processes used to manufacture and assemble the item as that can also provide an opportunity to reduce cost, which could be frosting on the cake for you, as a new engineer ;-)

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EX-Product 'Evangelist'
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[IRstuff]

Is this design lighter than a conventional lightweighted diaphragm? How thick is the existing diaphragm? What is the Pareto of weight distributions? You have
> diaphragm
> 6 angle bars
> 3-armed spoke
> fasteners

The main thing to ask is "what does this diaphragm do?" Is it decorative or functional?

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[drawoh]

dejan95,

MIL-STD-1472G states that a (man or a) woman can lift 14kg off the floor and place it on a shelf 152cm from the floor. A fifth percentile woman can lift a 10.5kg mass on a standing one[‑]handed pull. 12kg is well within the two[‑]handed lifting capabilities of a tiny woman.

Do some ergonomics. It looks like that thing has sharp, awkwardly placed edges. I suggest sturdy, comfortable handle positioned for convenient lifting.

MIL-STD-1472, Department of Defence Design Criteria Standard Human Engineering, is available free on the internet. Read it.

--
JHG

 

[hendersdc]

Good point drawoh, a couple well placed handles could make more of a difference to the operators than cutting down half the weight.

 

[BrianE22]

Where's the pulley, belt? I'm guessing the guard is used to prevent pinch points?

 

[rb1957]

this is actually a very good project for a beginner designer. There is a design process … follow it.

the questions hendersdc posted are a good place to start …

what is the function of this cover ?
Does the current design do this well ?
it is too heavy to move, or too awkward ?
can you change the design so that removing the belt doesn't require removing the entire cover (ie a fixed central cover, and a removable outer piece(s)) ?

This is a typical uni design project. Maybe read up some of your Mechanical/Machine Design texts ?

I'd go back with at least two proposals, just like choosing a house (one maybe be perfect, but too expensive; another really cheap but misses some key design features; another is a compromise; …)

another day in paradise, or is paradise one day closer ?

 

[rotw]

Make a new cover that is designed similar as a bike wheel (as the concept to ensure stiffness) and then put cover (on one side or both) which would be a thin plate (for protection)???

If you plan an escape, you must succeed as if you fail, you will be punished for trying. Never say or write down your plan. Heart is the only place where secrecy is granted.

 

[FACS]

This is a good answer:

Things I considered regarding weight:
- making parts from aluminum to reduce some weight; (Even Acrylic will work in some cases)
- reducing the diameter from 1070 mm to 950 mm (it would still serve the same propose); Good idea
- reducing the thickness from 3mm to 2 mm. Another good idea.

If that solve your weight issue, you are done.
After that, put in lightening holes.

 

[MintJulep]

Find the history of this design.

All those extra ribs were probably added because the original design was not stiff enough.

So going thinner may not be a good idea.

Do you understand how spoked bicycle wheels work?

 

[Compositepro]

The correct name for this part is a "spool flange". It keeps a roll of material from telescoping or otherwise deforming while unspooling under tension. The requirements are that it have a smooth flat surface against the material spool, and that it be stiff enough to stay flat. I have designed these before. It also appears that you have a "turret unwinder" holding two spools so that the next spool can be made ready and swung in to quickly replace the empty spool as soon as it runs out.

You may find that a better solution than lightening the flange may be to design a counter-weighted cable hook so that the flange can be raised and stored up out of the way while the material spool is changed.

If you still want a light weight material consider plywood with carbon fiber skins.

 

[BUGGAR]

Except for the flat surface requirement, I was thinking of a large spun aluminum disc like the Moon Discs of years ago. The dish shape gives it strength.

 

[dejan95]

Thank you all for your answers and advice!!

@Compositepro described the purpose of the cover better than I have. "It keeps a roll of material from telescoping or otherwise deforming while unspooling under tension."

I love @drawoh idea, about adding handles for more convenient lifting. But I will probably still try to reduce the weight because that was the primary request from the workers.

I return to work on Monday and I will do some additional research and let you all know what I found out.

Thanks again.

 

[rb1957]

be careful in slavishly following the "request from the workers".

Compositepro clearly understands how this is meant to work, and the requirements on the design.

Reducing the weight will take an extensive amount of engineering, so this'd be one option. I'd suggest Nomex core (say 3lbs per ft3) rather than plywood (very heavy). But making a composite cover creates other issues … environment, etc … nothing that can't be dealt with if it's recognised up front (but will be a royal pain if encountered later).

Is the real intent of the "workers" to make it easier to manipulate ? Then maybe a counterweight and/or handles are the other solution ?

Have you seen the guys working with this ? You should, to understand the problem.

another day in paradise, or is paradise one day closer ?

 

[drawoh]

dejan95

Make it big and forget it.

If you are designing shop equipment, you need to get it right on the first try, and you need to use up minimal engineering time in the process. You accomplish lightweight design by...

[dt]...reducing safety factors[/dt]
[dd]Take the time and effort to find out how this thing is used and loaded. Do the calculations. Factors of Safety are also known as Factors of Ignorance. You need to be less ignorant. Note rb1957's comments above on watching the guys using it.[/dd]
[dt]...using exotic materials and fabrication methods[/dt]
[dd]Carbon steel has an impressive cost to strength ratio. Make sure you understand your new materials and fabrication processes. Your lightweight design will cost more.[/dd]
[dt]...accepting failures[/dt]
[dd]Is it okay of this thing fails in service, or breaks when workers handle it? This is a cost and safety issue.[/dd]

Ergonomic design consists of you operating the device, and working out how it ought to operate. If you can imagine the movements you would like to make, you can work out the positions of the handles.

Maybe this thing needs to be heavy and sturdy. Engine hoists are cheap here in North America. I would be surprised if they were expensive where you are.

--
JHG

 

[GregLocock]

I can't help thinking that a hook and a springy-locky-cabley sort of thing (technical term) would get around the handling issue with no redesign.

Spring Balancer Retractable Tool Holder 15-22kg Hanging Equipment Yellow

Cheers

Greg Locock


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