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Lmu8 bearings and tolerances

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LT72884

Plastic
Joined
May 31, 2018
I am new to this whole machining/building scene and have a question. I purchased some lmu8 linear bearings for 8mm hardend rod. I know 8mm is 0.314in and 5/16 is 0.312in. Since i do not know tolerances to much, would a 2-thousandths of an inch be to much of a difference for the bearings to run smooth?

Thanks

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LT72884 said:
I am new to this whole machining/building scene and have a question. I purchased some lmu8 linear bearings for 8mm hardend rod. I know 8mm is 0.314in and 5/16 is 0.312in. Since i do not know tolerances to much, would a 2-thousandths of an inch be to much of a difference for the bearings to run smooth?

Thanks

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Click to expand...
Your an engineering student, I would ask your professors for help on
"tolerance to much"

If you don't get at least a semester in GD&T, demand your tuition back.
 
digger doug said:
Your an engineering student, I would ask your professors for help on
"tolerance to much"

If you don't get at least a semester in GD&T, demand your tuition back.
Click to expand...
Have not had any classes on this and im almost done with school. Doubt any of my professors know. Had a BASIC intro class to machine process but nothing on gd&t



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Many things related to bearings have tolerances in the +/- .0002" range....

Can't say for sure what you've got won't work... But .002" is a MILE in bearing fits.
 
Well you probably had a basic intro to GD&T in CAD or drafting class, or similar, they just don't call it that specifically. I'm not surprised you didn't get a dedicated class. Regardless it wouldn't have helped you in this specific application.

The bearing supplier should be able to provide info on the tolerances, if not mcmaster carr's website is a good resource. What I found is they don't really give much, but they are designed for 8mm "shafting", which implies a level of quality. Most shafting is spec'd +0, -xyz where xyz is something very small like ). For 8mm it's 0.009mm or roughly 0.0004" (again undersized only).

You're asking about more like 0.003" undersized (8mm is closer to 0.315") assuming you're talking still about precision shafting and not hot rolled rod from the hardware store. Basic rod may be oversized and not work. It also likely won't be round enough.

It really depends on what you're doing/making. It's not a particularly sloppy fit and could work just fine for something low friction in your garage with pieces left over. But mixing the two isn't good practice unless there's a specific reason to. Sure it'll slide through, but it's nothing you'd sell, submit for a grade, etc.
 
TeachMePlease said:
Many things related to bearings have tolerances in the +/- .0002" range....

Can't say for sure what you've got won't work... But .002" is a MILE in bearing fits.
Click to expand...
Now thats interesting to me. I mean, in my brain, its hard to comprehend 0.002inch difference. Thats so small to me.

Im just using it for my x-axis of a 3d printer im building.

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JCByrd24 said:
Well you probably had a basic intro to GD&T in CAD or drafting class, or similar, they just don't call it that specifically. I'm not surprised you didn't get a dedicated class. Regardless it wouldn't have helped you in this specific application.

The bearing supplier should be able to provide info on the tolerances, if not mcmaster carr's website is a good resource. What I found is they don't really give much, but they are designed for 8mm "shafting", which implies a level of quality. Most shafting is spec'd +0, -xyz where xyz is something very small like ). For 8mm it's 0.009mm or roughly 0.0004" (again undersized only).

You're asking about more like 0.003" undersized (8mm is closer to 0.315") assuming you're talking still about precision shafting and not hot rolled rod from the hardware store. Basic rod may be oversized and not work. It also likely won't be round enough.

It really depends on what you're doing/making. It's not a particularly sloppy fit and could work just fine for something low friction in your garage with pieces left over. But mixing the two isn't good practice unless there's a specific reason to. Sure it'll slide through, but it's nothing you'd sell, submit for a grade, etc.
Click to expand...
Thanks for this info. Here is the thing... mechanical engineering degrees do not require ANY cad classes at all. They took them all out. Their reasoning is that drafters, not engineers, design the parts.

In fact, i have had to look at outside tech schools for a solidworks course which ill be taking. I asked the professor if we do any gd&t stuff for the cswa and he said nope.

This is why i am building a 3d printer. I need hands on exp and this is the best place for me to start.

One university here, gives very simple hands on projects, but its like highschool stuff. Nothing like wing design, machine design, or anything like that.

The 5/16ths rod is plain steel from homedepot. I did find bearings that will work on either 8mm or 5/16ths, but how out-of-round the homedepot stuff is??? Thats the question haha

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LT72884 said:
Thanks for this info. Here is the thing... mechanical engineering degrees do not require ANY cad classes at all. They took them all out. Their reasoning is that drafters, not engineers, design the parts.

In fact, i have had to look at outside tech schools for a solidworks course which ill be taking. I asked the professor if we do any gd&t stuff for the cswa and he said nope.

This is why i am building a 3d printer. I need hands on exp and this is the best place for me to start.

One university here, gives very simple hands on projects, but its like highschool stuff. Nothing like wing design, machine design, or anything like that.

The 5/16ths rod is plain steel from homedepot. I did find bearings that will work on either 8mm or 5/16ths, but how out-of-round the homedepot stuff is??? Thats the question haha

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Click to expand...


McMaster-Carr

An 8MM x 600MM is $21... Just buy the right stuff.
 
I guessed 3D printer before you mentioned the use, just from the size of the bearings. But I was a little disheartened to see you not comprehending the impact that even .002/.003" would have for this use.

The reason it matters is the bearings you're using are linear ball bearings, using very tiny balls (~1mm or so) to contact a rail. The balls are very hard, and while the loads are low, the contact force is not, especially under repeated accel/deccel loads with an asymmetric force applied to them. That is, your platform does not have the force applied at the center of mass, therefore there's an overturning force of some amount every time the platform is moved.

This puts high contact force through tiny contact patches, and when using soft rails and hard bearing balls means the balls "dig into" the rails, both creating friction and causing wear. This also introduces a skew/slew to the platform, exacerbated with any play (clearance or wear), so displacing the platform.

Displacement means inaccurate deposition, which is the bane of 3D printers. If you want to make a good one, you need to correctly spec the bearings, support rails, and structure for maximum stiffness and minimal weight, and try to have the applied force going through the CG of any moving elements to minimize displacement errors. This includes drag loads from plastic feedstock, electrical wiring, etc.

Not to be harsh, but where are you in your educational path? If you're near graduation, I would strongly encourage you to look for additional hands-on opportunities with someone who's actually designed and built things. I'd also question how good the curriculum is in your school, as the lack of understanding you evidence is worrying. Again, not to slam you, but to point out there's a lot of gaps shown by your questions.

A note - I'm a part-time instructor at a college shop, and I've seen some very bright students and some really questionable ones come through the doors. Take what I'm saying as encouragement to keep asking questions, but also to dig into other sources (including data available online from bearing suppliers) to further teach yourself.
 
LT72884 said:
Thanks for this info. Here is the thing... mechanical engineering degrees do not require ANY cad classes at all. They took them all out. Their reasoning is that drafters, not engineers, design the parts.
Click to expand...

And this right here makes me wonder if you can transfer to a school that actually, I don't know, TEACHES mechanical engineering?? Good gosh, this does help explain why Boeing can't build planes, Ford can't build cars, and GE can't do anything right anymore.

I can't tell you just how frustrating it was to read your sentence. Let's just say I'm glad I wasn't in earshot of the person who told you that.
 
Milland said:
And this right here makes me wonder if you can transfer to a school that actually, I don't know, TEACHES mechanical engineering?? Good gosh, this does help explain why Boeing can't build planes, Ford can't build cars, and GE can't do anything right anymore.

I can't tell you just how frustrating it was to read your sentence. Let's just say I'm glad I wasn't in earshot of the person who told you that.
Click to expand...
Im almost done with the bachelors of engineering degree.

The classes i have had i have learned a tremendous amount from, but its all been theory. I have had 13 semesters of math, 3 of physics, 2 of chem, thermo, statics, material science, dynamics, strengths, vibrations, programming, matlab, and a couple more classes. Dont get me wrong, my professors are awesome and i have learned a ton, but lack the understanding of design. Not one single class has taught me the difference between a bushing or bearing haha. I had to look that up today.

This is why i had to look outside of the universities, all 5 of the major ones here, to get a basic solidworks course which still doesnt cover much.

I have had to do ALOT of my own research. Believe me, 4 months ago... i didn't even know what a stepper motor was, nor the fact of why its called that. I do now, but not 4 mo ths ago.

I know basics of threads and springs, but im thinking i need to get the machinists handbook? I think thats what it is called.

Anyway, thanks for help. I did know that i had to use hardend steel because of the way the bearings dug into it the rods after time. I just didnt know that they were that tightly toleranced and just by being 0.003 difference between the rods, caused that much "play" between bearing and rod.



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From the classes you've taken I'd say you had a great start towards becoming a physicist, but not a functioning ME. There's almost nothing there that would give you a practical understanding of design of mechanisms and systems except on a conceptual level, which is helpful but not the same as getting down and dirty with machines and finding out what works and what doesn't.

I spent a couple years aiding a FSAE team, and it was a great platform for getting real knowledge and understand of ME processes. I wish you could have done something like that in your system...
 
Milland said:
From the classes you've taken I'd say you had a great start towards becoming a physicist, but not a functioning ME. There's almost nothing there that would give you a practical understanding of design of mechanisms and systems except on a conceptual level, which is helpful but not the same as getting down and dirty with machines and finding out what works and what doesn't.

I spent a couple years aiding a FSAE team, and it was a great platform for getting real knowledge and understand of ME processes. I wish you could have done something like that in your system...
Click to expand...
I agree. I have looked at schools out of state amd same thing. I talked with a dean in new mexico and he said they used to have really good hands on classes but fed gov has really pushed hard to get rid of them. Not teying to start a political convo here, it could be true or not.

All the ME degrees in my state and surrounding states are all the same. Tons of theory, physics, and math, but no design or hands on.

But i still have a year left, maybe they will change things up.

What do you recomend as some good research and hands on?

Thanks



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Heck, this is dreadful if there's nothing like First Robotics, FSAE, or other "Mech" club-like activities you could join at your school.

OK, this'll sound stupid, but it can be very educational. If there's nothing organized in your area, then go to thrift shops and buy some things, like a toaster, oscillating fan, old (mechanical) typewriter, and things of that nature. Nothing over $10.

Also get yourself a good, basic toolkit, including a range of quality screwdrivers, pliers, smaller socket set, etc. And a good DVM (digital volt meter). Don't skimp here, as these should be lifetime tools if treated well.

Now start taking things apart. Look for how the parts are designed, how they're assembled (and so disassembled), and what materials are used for what components. Examine the design choices, why certain fasteners were used, or snap fits, or adhesives. Check out the mechanisms, like a bimetallic thermostat and "lift control" on the toaster. See how the oscillation mechanism on the fan works, and why it's an effective parasitic add-on to allow driving the fan from side to side.

It's old-school for sure, but diving deep into the bits will help you understand practical engineering and design much better than just book learning.

And once you've taken the items apart, put them together again and see if they work. And if they didn't, see what it takes to fix them. Add a soldering iron to your tool kit, but always be very careful when plugging something in for the first time. Check with the DVM for shorts in a power cord or similar "bitey" risks.

If there's a local repair shop or even an auto club of some form, perhaps try to join in an intern-like mode, and help out the expericanced mechanics while asking questions. Most guys/gals are willing to teach if they know you're serious about trying to learn. Or repair bicycles for a local charity, anything to get your hands dirty while the mind is engaged.

Hope this helps - the best students I've worked with have had a balance of classroom and hands-on learning, and many have gone on to start companies and design some very cool stuff.

And wear safety glasses if you don't wear specs to begin with. Some springs or sparks may target your eyeballs, and you really want to keep those in good shape for as long as possible.
 
Milland said:
Heck, this is dreadful if there's nothing like First Robotics, FSAE, or other "Mech" club-like activities you could join at your school.

OK, this'll sound stupid, but it can be very educational. If there's nothing organized in your area, then go to thrift shops and buy some things, like a toaster, oscillating fan, old (mechanical) typewriter, and things of that nature. Nothing over $10.

Also get yourself a good, basic toolkit, including a range of quality screwdrivers, pliers, smaller socket set, etc. And a good DVM (digital volt meter). Don't skimp here, as these should be lifetime tools if treated well.

Now start taking things apart. Look for how the parts are designed, how they're assembled (and so disassembled), and what materials are used for what components. Examine the design choices, why certain fasteners were used, or snap fits, or adhesives. Check out the mechanisms, like a bimetallic thermostat and "lift control" on the toaster. See how the oscillation mechanism on the fan works, and why it's an effective parasitic add-on to allow driving the fan from side to side.

It's old-school for sure, but diving deep into the bits will help you understand practical engineering and design much better than just book learning.

And once you've taken the items apart, put them together again and see if they work. And if they didn't, see what it takes to fix them. Add a soldering iron to your tool kit, but always be very careful when plugging something in for the first time. Check with the DVM for shorts in a power cord or similar "bitey" risks.

If there's a local repair shop or even an auto club of some form, perhaps try to join in an intern-like mode, and help out the expericanced mechanics while asking questions. Most guys/gals are willing to teach if they know you're serious about trying to learn. Or repair bicycles for a local charity, anything to get your hands dirty while the mind is engaged.

Hope this helps - the best students I've worked with have had a balance of classroom and hands-on learning, and many have gone on to start companies and design some very cool stuff.

And wear safety glasses if you don't use specs to begin with. Some springs or sparks may target your eyeballs, and you really want to keep those in good shape for as long as possible.
Click to expand...
That i can do. I live right across the street from a thrift store. I can pick up some basic items and see how they work. Gotta start somewhere.

As for tools, i JUST got a nice precision screwdriver set, soldering iron, fluke dvm, wrenches, sockets, and a grinder haha.

Slowly been adding to my collection.

The reason im building a 3d printer, i purchased a cr10 a few months back and it was all pre-built, but i wanted to learn how they work. So i watched all the assembly videos for the prusa 3 and now im building one haha.

Thanks for the help. I agree with the charity aspect of it.

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car2 said:
I don't know what decade the instructors, professors and administrators are working in, but "engineers" since the 80's started being responsible for design, drafting, testing, documentation, and often secretarial and purchasing "duties". When I first started working in early 1980 in engineering at IBM, they, and other companies had all sorts of "support" and "service" groups and resources, that quickly faded away to doing a good part of every aspect of a project/task yourself.

Anyway, don't worry, as long as you're interested and have the aptitude you'll pick things up; the key is listening and learning from people who know things. I didn't know much in detail about machining and design when I started working (and knew I didn't know much). I'd go down to the big model-shop right down the hall, often with a box-o-donuts in hand, and get the machinists to critique my designs/ideas--I learned an immense amount of practical info from them, and they enjoyed it also. I could get entire "under the table" projects done with no purchase-requisitions, whereas some of the "engineers" who considered the toolmakers the "hired help laborers", couldn't get a hole drilled without a purchase order. Been collecting tools and stuff for 40 years, so you have plenty of time.

***Get a copy of "Machinery Handbook"***, that will tell you everything you'd like to know about bearings and different types of fits and tolerances, along with a great deal of other critical practical design and reference info. I don't know why that isn't a "required book" for mechanical engineers. Cheers.
Click to expand...
Machinery handbook! Thats the one i was lookin at. Ill find a used copy somewhere.

Thanks for all the input. Im happy to be a part of this group haha.



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