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    Discussion in 'The main mechanical design forum' started by yadnom1973, Sep 16, 2016.

    1. yadnom1973

      yadnom1973 Member

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      Hi.

      I’m working on a design for a small rail based camera dolly. The dolly holds only the camera on a small carriage running along two parallel 42mm aluminium poles supported on scaffolding brackets. It differs from the normal dolly of this type in that it has as simple set of gears and a fly wheel inside to generate inertia and make the camera moves more fluid. The basic structure is three vertical, parallel sheets of aluminium with three shafts running between them. One carrying the flywheel, one carrying gears and the last the axel of the two front wheels. I need to keep it as quiet as possible so I’m using nylon helical gears and polyurethane wheels. The shafts are mounted in bearings where they pass through the aluminium panels.

      This is the first time I’ve built something like this so I thought I’d try to find somewhere to try and ask some advice about what I’m doing.

      The things that concern me are quite simple but I just can’t find out how I should go about them.

      Fixing the bearings in the panels. The only way I’ve found of mounting the bearings so far is folk freezing the bearings and heating the panels then pressing the bearings in and when it cools they should be fixed in the panel tight. Is this a good idea, I don’t like the idea of heating aluminium, is there a better way of going about this?

      The other thing is that if the axel passes through the two panels supported in the bearings how do I prevent the axel sliding out of place. I thought of putting collars onto the shaft either side of the bearings but I don’t want any friction with the collars rubbing against the panel creating drag and noise. So I thought of putting spacers between the collar and the bearings so they grip only the central part of the bearing but now I’m concerned that pressure from the side on the bearings, if the wheels get a knocked from the side for example during transport, that this might damage them, are they designed for the kind of thing? The shaft at the moment is 12mm and the panels 10mm but this can be changed if it would solve the problem.

      Anyhow I can’t help thinking that this is a very common situation and there must be some standard way of going about it. I could sandwich the bearings between two sheets with recesses milled into them but I’d like to find a more simple solution. But I’ve not managed to find it anywhere so I thought I’d come here and ask if anyone could help, or if I could just be pointed in the direction of somewhere I could learn this stuff.

      Thanks, N.
       
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    3. Dana

      Dana Well-Known Member

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      A shrink fit with heat is not what you want except in very special situations, i.e. very large bearings.

      Typically ball bearings are press fit into one component and slip fit with the other. Which is which depends on the application. Most often the press fit is on the component where the load rotates relative to that component. In your case, it's the shaft, so you would want a press fit on the shafts and a slip fit in the side plates. Most of the better bearing manufacturers publish tables of the recommended shaft and housing fits for their bearings.

      Assuming the side plates are thicker than be bearings, you will likely want to machine a counterbored hole for the bearing, which will hold the bearing on one side. Again, the bearing manufacturer's tables give recommended sizes for the through hole to avoid interference with the rotating part of the bearing. On the shaft, depending on the load a the press fit might be enough to hold it in place, or you might need a shoulder on the shaft (which can be a machined shoulder or some type of retaining ring). Assuming your bearings are on the inside of the plate so they can't move outward, you'd assemble the bearings and other components onto the shafts, and then slide the side plates into position, which keeps everything in place.

      You can also get flanged bearings, which can go into a through bored hole in the side plates, with no need to machine a counterbored hole. If the flange (or counterbore) is on the outside then you would need a retaining ring or something outside to hold everything together.

      Or you can get flanged "pillow block" bearings, which are already mounted into a housing which you simply bolt to your side plates. These can be had with a set screw or other locking arrangement to hold the shaft in place.
       
    4. yadnom1973

      yadnom1973 Member

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      Yes, the plates will be as thick as the bearings. I’ll do some research on the bearings to find the relevant information you are talking about but it all sounds reasonably painless now you have explained it. I’ve been staring at keys in shafts and liquid nitrogen prices all night.

      I’ll machines the holes for the bearings on the inside and then hopefully the bearings will hold the shaft tight enough. I might try and cut a little groove into the shaft and drop a circlip in there to retain the bearings in place just to be safe. Thanks for taking time to answer my questions it’s given me the confidence to finish off the drawings and start ordering the parts.

      All the best. Neil.
       
    5. K.I.S.S.

      K.I.S.S. Well-Known Member

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      I've some experience in the film industry, basically re-modelling and re-engineering parts for friends who supply 'grip' equipment. All as a favour. ..
      From experience, if you want a fluid flow to the movement of the equipment, use a fluid. This usually entails the employment of a very viscous oil, normally in the region of 100,000 Centistokes, and directed around variable channels in order to accurately and seamlessly control movement.
      If you're interested to know some more, just let me know
       
    6. yadnom1973

      yadnom1973 Member

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      Thanks, I'm just getting my head around solidworks to get the drawings done but once they are I'll put them up and see if anyone can see any obvious mistakes. My main concern is with tolerances, the mod of the gears and the thickness of the metal parts and whether they are the appropriate for the weights involved.

      I think you are talking about the fluid heads on tripods, that would be a very advanced project for me I'm afraid, this is a dolly for tracking shots that I'm making and though a fluid head would be mounted on top of it when in use it's just a little go cart really.
       
    7. yadnom1973

      yadnom1973 Member

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      actually if you fancy giving some advice I am trying to find a way to secure aluminium poles that slide through the dolly. They need to be taken in and out depending on the use so I have holes passing through either side of the carriage and to secure them I was thinking of cutting a slot and drilling and tapping a hole to make pinch fittings.

      The sides are 10mm thick though they could be made a little wider and the poles are just under 27mm diameter. here's a little picture to help explain.

      [​IMG]

      Do you think this will work? I've seen similar things as a way of securing tubing but not in aluminium.
       
    8. yadnom1973

      yadnom1973 Member

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    9. K.I.S.S.

      K.I.S.S. Well-Known Member

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      I always try to be positive, but I do have to ask this - are you trying to make a cheap dolly, because the rental rates per day in the film industry are exorbitant?
      If that's what you're trying to achieve, as a commercial venture, then you're on a hiding to nothing. ..
      Commercial dollys also make use of hydraulics, whereby the unit must be capable of 5 full raises and depressions under a single battery charge. It's an extremely moribund industry, but it is what it is. I've had a good few ideas about how to simplify and improve the dolly, but the industry isn't interested. From an engineering perspective, gears are clunky and gearboxes magnify this, and this is exacerbated when projected on a large screen.
      And yes, you were correct in thinking I was talking primarily about fluid drive heads, Ronford Baker mostly.
      Cheers
       
    10. K.I.S.S.

      K.I.S.S. Well-Known Member

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      Sorry, I didn't mention this before, but you show what appears to be an extremely small moment in the sketch. You basically have a 3 to 1 ratio between the tube diameter and the supporting material. That's not good. It will almost guarantee certain failure modes, depending on function. If static, as you indicated, it will promote 'skewness' and if sliding, it will promote 'sticktion', a condition where the combined coefficients of friction of the two components will leap off the scale.
      Reverse the ratios of your diameters and thicknesses to get something smooth.
      Cheers
      John
       
    11. yadnom1973

      yadnom1973 Member

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      I'm not making a fluid motion system of any kind and I'm not trying to build a dolly as you are thinking of one with hydraulic jib arm and such. What I'm making is a small carriage that runs along rigged aluminium rails, like a slider that can be extended to several meters, underslung and used in many other ways. The wheels are geared to a flywheel to create inertia and smooth acceleration. It can produce very constant slow speed pans and such. I'm not reinventing any wheels just taking a few of the best functioning systems about and adapting them into one for my specific use. The carriage is a aluminium box of roughly 35cm x 25cm and about 12mm thick walls. the aluminum tube slides through one side and out the other, it passses through both sides but the picture is of just one side and the idea to have a pinch fitting to lock the tube in place. The carriage moves on wheels down the rales, nothing else slides or moves in any way. The sketch is not to scale in any way I was just trying to clarify what I was suggesting.

      I'll have some drawings done soon, that will clarify what I'm doing much better.
       

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