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Old Mar 15, 2013, 03:39 AM
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again i will attach photo of mechainsm i want to avoid any more confusion and repeat the piston will be cycling against gravity
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Old Mar 15, 2013, 03:41 AM
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here is the mechanism again
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Old Mar 15, 2013, 05:45 AM
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gouch first of all man chillout, i didnt take balancing in uni so i dont exactly know wat counter balancing is for. second of all i came here to ask for advise about motor and how to control it and power it but not about the design itself;i know wat im doing so dont worry about it. we are now past the motor issue as nereth and tim showed me the best option to save time which is dc brushed geared motors. u misunderstood wat i need the linear bearings for but that is not important to our topic.I changed my mind to use lesser load cause i realized 25 kilos at 200 will need a big motor unsuitable for my design and will be more expensive as well. im now going to take some days to read about info nereth attached and come back to him if he is still willing to help.

one last thing gouch, which high school student would make such a mechanism that will cost several hundreds of dollars for a silly high school project!!!
I'm cool as fonz mate, don't worry about me.

I was interested with your project until you said you were a Mechanical engineer. That's when I started to chuckle after some of the things you have written, especially when you showed your skill on the topic of "what bearings actually do"!

You come across as a high school kid who didn't learn the English language, U no wat I mean? that's why I posted the school project quirk.

You don't have to take "balancing in uni" to know that 2KG lump hanging off a rod and crank setup that is doing 200 RPM is going to need something countering the forces. Simple IC engine design programs you can find online (cause that's basically what it is except for your funky linear bearings) will give you all the info on the forces at work here. Having said that you should already know the forces involved. That 2KG lump (plus pin and a % of the rod/) will do 3.3 odd cycles a second at 200 RPM. that's a fair amount of mass to accelerate, stop and drag it back again reasonably quickly, time and time again. Sure 200 RPM aint no F1 engine RPM, but it's going to need something to stop it rattling itself to pieces pretty quickly

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a bearing attached to the output shaft of my motor will be enough to reduce vibrations. if u think other wise please tell me.
That's gold! but hey, why would I tell you otherwise, you're the mechancial engineer I'm just a mechanic, but if I put that down in a first year exam, my teacher would have beaten me with a rubber hose.....and rightly so!

btw, the motor is only there to drive the thing, it should not be used to counter poor mechanical design

I truly wish you the best of luck on this top secret project.....whatever it's for.
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Old Mar 15, 2013, 05:54 AM
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Which vibrations are you talking about Gouch? Vertical ones in the axis of the piston, speed oscillations, or miscellaneous extras?
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Old Mar 15, 2013, 05:58 AM
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It's a funny thing about us Engineers some become uber specialized and miss parts of what might be considered 'normal'. I did Electrical but finished up using a hybrid of Mech/Chem with some Elec on top for a job before I took to playing with Toys

The Best Engineers I have worked with have always been the diploma qualified ex trade types from the mines or heavy industry. Practical and new how to cut through the theoretical BS and get a working solution
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Old Mar 15, 2013, 06:42 AM
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Which vibrations are you talking about Gouch? Vertical ones in the axis of the piston, speed oscillations, or miscellaneous extras?
all of 'em!

All it really needs is a flywheel with a counterweight opposite the "crank" pin, just like any engine. Building something like this without any decent balancing is one of the main reasons old triumph motorcycles used to fall apart as you rode down the road!

here's a test you can try to see if it needs balancing...... If you have one of those reciprical saws, try putting even only 1KG on the end of the blade and turn it on to it's lowest setting ......hold on nice and tight though!
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Old Mar 15, 2013, 06:50 AM
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all of 'em!

All it really needs is a flywheel with a counterweight opposite the "crank" pin, just like any engine. Building something like this without any decent balancing is one of the main reasons old triumph motorcycles used to fall apart as you rode down the road!

here's a test you can try to see if it needs balancing...... If you have one of those reciprical saws, try putting even only 1KG on the end of the blade and turn it on to it's lowest setting ......hold on nice and tight though!
Well I mean with some big, good quality radial needle bearings and tight tolerances, or maybe just bushings, held in a (heavy) steel or cast iron structure and bolted to a granite slab (or just more heavy steel or cast iron for weight) on a concrete factory floor with some rubber vibration isolation here and there on the mounts so nothing fractured, you could probably do it without counterbalancing for a while.

Or you could counterbalance it.

But I quite enjoy the "challenge accepted' approach to personal engineering projects so I say let's do it.
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Old Mar 15, 2013, 07:17 AM
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or you could counterbalance it.
...
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Old Mar 15, 2013, 12:50 PM
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gouch i did suspect u were not an engineer after wat u said. as tim pointed out, engineers have fields of speciality in uni and after uni, so we dont remember or are not taught some things that seem normal to u. besides as mechanical engineers we are taught theoritical general stuff.some diplomas on other hand are taught practical stuff and can work on related jobs like an expert right away. in my new work at the shipyard the mechanical forman in my department actually thought i faked my certificate; he took me aside and asked if i actually got a degree and promised he wouldn't tell any one if i told him the truth

regarding bearings i do know wat they are used for which is reduce friction; i just thought that some bearings with tight clearance can stop the rods from shaking.

finally, i will have to open my kinematics of machinary and read the balancing chapter WHICH WE WERE NEVER TAUGHT BEFORE
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Old Mar 15, 2013, 01:18 PM
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If you want to run continuously at a single speed, how accurate, and how consistent does that speed need to be?

You could literally plug a battery straight into it, but there would be voltage sag over time and you wouldn't be able to precisely control speed other than by changing battery chemistry and cell count.

If you want consistent output you need a source that isn't a battery. You could hack one together with a variac and some caps but I'm pretty sure that isn't the direction you want to go with this, haha =D. You would need to buy a DC power supply with sufficient amps and volts to run the experiment. You need to choose a motor and drive system to know how many amps and volts you need. You may already have a DC power supply around.

Actually the lead acid setting on one of our hobby chargers of sufficient rating (say 200W) would also work I imagine depending on how high a voltage it lets you set (my little one next to me right now only goes up to 20V). Not sure if it would be cheaper or more expensive, or how reliable it would be compared to just getting a DC power supply.

Neither of these would give you automated, precise speed control (although you might be able to hack it into a DC power supply), but speed should be relatively constant assuming your load isn't changing, so you can likely get it close enough with trial and error with a optical tachometer and your hand on the voltage knob of a DC power supply, and then just leave it there.
nereth i read about voltage sag but wat makes u think it will happen. how long does it take to start saging. i need the speed to as precise as possible. the speed should be consistant throughout the whole cycle.
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Last edited by cloudsofwar; Mar 15, 2013 at 01:20 PM. Reason: add info
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Old Mar 15, 2013, 01:21 PM
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How big of a battery are you using? And what chemistry?

If you use a battery that could last 24 hours at your output and then only run it for a few minutes you'll probably see negligible sag.

If you use a lipo that will last 1 hour without damage and use it for a full hour, you will probably see 20% or so sag.

Batteries experience voltage sag as they discharge through their capacity. If they directly drive something (no DC-DC converter involved) It very nearly directly correlates to a speed sag (slightly less than linear relationship I would imagine)
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Old Mar 15, 2013, 01:43 PM
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gouch i did suspect u were not an engineer after wat u said.
you mean the bit where I said I wasn't a mechanical engineer? I can see why you would suspect it then

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regarding bearings i do know wat they are used for which is reduce friction; i just thought that some bearings with tight clearance can stop the rods from shaking.
you can not control vibrations by fitting "tighter bearings". think about it. All you are doing is creating a failure point.

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finally, i will have to open my kinematics of machinary and read the balancing chapter WHICH WE WERE NEVER TAUGHT BEFORE
my point was as mech eng is your qualification, one would have assumed you would have already done the numbers and figured this out by now......that is my whole point! Sure they may not have taught it to you, but when it comes time to design something serious and big at the shipyard, and it rattles itself into oblivion, or breaks because you didn't run the numbers because you weren't taught it..... you can't be blaming your teacher!


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in my new work at the shipyard the mechanical forman in my department actually thought i faked my certificate; he took me aside and asked if i actually got a degree and promised he wouldn't tell any one if i told him the truth
honestly, I can see why he would say that!

Look, jokes aside, I'm really interested in why you think this design is the best way to do what you want to do........whatever it is for . I don't understand why you need something to rotate like this when all it is doing is lifting something up and down. Don't worry, I'm certainly not out to steal your idea, I just find it an odd design. Maybe you should see what industry is using to do similar things nowdays rather than building this contraption.

One other thing I thought of: I assume you have factored in side thrust on one side of the linear bearing for your piston. The sideload comes from the rod imparting a side thrust on the piston. In engines, they usually offset the centreline of the crank to minimise this side thrust, rather than having it on the centreline as you have drawn it. If your bearings can handle it, then no problem of course, just throwing it out there.
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Old Mar 15, 2013, 03:51 PM
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...
I am sure I suggested that about 30 posts ago too
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Old Mar 15, 2013, 03:57 PM
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indeed you did, but I thought i'd revisit it, as it was forgotten about fairly quickly
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Old Mar 15, 2013, 04:10 PM
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This is true seems lots of good practical roughly engineered stuff that will work has been forgotten or lost in the novelish posts that followed
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