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Thread: Clutch' Use in this Gearbox

  1. #1
    Associate Engineer
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    Clutch' Use in this Gearbox

    Hi there,

    We are asked to design this helical gearbox download.gif that has an electromagnetic clutch in it using given values as our term project. But I don't get what is the clutch supposed to do here. How does it engage or disengage power transmission in this gearbox? Also can you tell me what are the parts that I marked?

    Thank you.

  2. #2
    Administrator Kelly Bramble's Avatar
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    Are you serious?

    What other information do you have, specifications, parts list, 3D model..
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  3. #3
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    Only input power, shaft speed and gear ratio and desired bearing life are given. Clutch attached to input shaft and pulley are not calculated/unimportant. Basically we are asked to design a gearbox from scratch. Calculations are not required to be too detailed or specific as long as you end up with something within a reasonable range and something that is supposed to be working. There is no parts list nor 3D model. We are asked to select parts like screws, fasteners, bearings, rings etc as we see fit. There are no specifications related to EM clutch.

  4. #4
    Administrator Kelly Bramble's Avatar
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    You need a concept first, not sure why you have a cross section from which to work with.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  5. #5
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    I honestly don't know about the cross section either but I'd like to know what is that em clutch supposed to achieve here.

  6. #6
    Technical Fellow jboggs's Avatar
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    From what I see there you have an input shaft in the upper left driven by a flexible coupling. The driver gear on the input shaft engages with the driven gear on the output shaft. The driven gear is mounted on, but not directly connected to, the output shaft. It is free to spin around the output shaft on the plane bearing in the lower red box. The clutch is also mounted on the output shaft, to the left of the driven gear. The clutch output is firmly connected to the output shaft through a key. The clutch input is connected to the driven gear through the arrangement shown in the upper red box. The output shaft drives a 6V V-belt pulley. This arrangement is unusual in that the clutch is internal to the gearbox.

    The clutch allows the input shaft to rotate without driving the v-belt pulley, a very common requirement. You will find that many times equipment may need to start and stop periodically. It can be very difficult if not impossible to start and stop most prime movers (motors, etc.) like that. That's why clutches are needed. When you stop at a red light in a straight drive car you have to disengage the clutch to allow the motor to continue to run.

  7. #7
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    Jboggs is correct, the electromagnetic clutch in the gearbox is unusual.

    The first step in your redesign is to move the clutch to the input shaft on the exterior of the gearbox. The input shaft has the higher speed and lower torque. The lower torque means that the torque capacity of the clutch can be lower and thus the size, cost and electrical power consumption will be lower. The heat from the electric current in the clutch will not be directly inside the gearbox.

    Helical gears generate an axial force component that clutches aren't built to handle in most cases.

    Note that the clutch is now mounted on a flat, presumably machined surface inside the gearbox. Machining that surface, which is larger in diameter than the bearing hole on the output side is not as easy as machining consecutively smaller diameter. Moving this surface to the outside will make manufacturing easier. Eliminating the routing of electrical wires into the gearbox will also help.

    Next get rid of at least half of the bearing end caps and put the seal counterbores on the outside where the maintenance people can change them without removing the caps.

    The undercut area in the bottom of the gearbox requires a core to be set during casting. Eliminate the need for a separate core if you can. The cover does not seem to serve a structural purpose. Replace it with a flat plate or stamping and put the fill and drain plugs on the side. You are machining from those sides already.

    Ball bearings are not the best for handling the axial loads of helical bearings.

    Are two small bolts better for retaining the pulley than threading the end of the shaft for a nut?

    Tell us what kind of grade you get.

  8. #8
    Principle Engineer Cragyon's Avatar
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    I'm with Kelly, how does a design cycle even work when given a cross section image and nothing more?

  9. #9
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    Quote Originally Posted by Hudson View Post
    Jboggs is correct, the electromagnetic clutch in the gearbox is unusual.

    The first step in your redesign is to move the clutch to the input shaft on the exterior of the gearbox. The input shaft has the higher speed and lower torque. The lower torque means that the torque capacity of the clutch can be lower and thus the size, cost and electrical power consumption will be lower. The heat from the electric current in the clutch will not be directly inside the gearbox.
    Clutch is supposed to be inside the gearbox. We can change/decide bearing, gearbox may be. Maybe it's like this due to spacing issues idk.

    Quote Originally Posted by jboggs View Post
    The driven gear is mounted on, but not directly connected to, the output shaft. It is free to spin around the output shaft on the plane bearing in the lower red box.
    I assume plain bearing is press-fitted to the gear hub and there is a lubricant layer between bearing and the shaft so gear can rotate without shaft rotating. Is that correct?

    Quote Originally Posted by jboggs View Post
    The clutch is also mounted on the output shaft, to the left of the driven gear. The clutch output is firmly connected to the output shaft through a key. The clutch input is connected to the driven gear through the arrangement shown in the upper red box.
    What type of fastener can be used for that of arrangement? What is the one used in the drawing?

    Thank you all for your help!

  10. #10
    Technical Fellow jboggs's Avatar
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    Quote Originally Posted by oyyunnnn View Post
    Clutch is supposed to be inside the gearbox. We can change/decide bearing, gearbox may be. Maybe it's like this due to spacing issues idk.
    Three questions: (1) As Hudson pointed out, one would expect to see the clutch on the high-speed, low-torque input shaft. Why isn't it there?
    (2) Normally gears in a reducer like this operate in an oil bath. That would put the clutch in the same oil bath. I doubt that is the case. So, how is this reducer lubricated?
    (3) Why is the clutch "supposed to be inside the gearbox"? That means you have to design a clutch to fit and operate there. Normally one would see a commercially available, off-the-shelf clutch mounted somewhere on the input to the gearbox. What is the operational requirement that mandates that a specially designed clutch must operate somewhere in or near an oil bath inside the gearbox?

    Quote Originally Posted by oyyunnnn View Post
    I assume plain bearing is press-fitted to the gear hub and there is a lubricant layer between bearing and the shaft so gear can rotate without shaft rotating. Is that correct?
    Yes. There are several options for lubrication depending on operational requirements (oil bath, impregnated bearing, external lubrication). Don't forget the need for alignment/thrust bearings too.

    Quote Originally Posted by oyyunnnn View Post
    What type of fastener can be used for that of arrangement? What is the one used in the drawing?
    It is really hard to tell from the figure. Zooming in it appears to be some kind of pin that goes all the way through the gear and the clutch drive plate and is flared on each end, kind of like a rivet. A threaded fastener might work with some redesign.

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