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Thread: Crown and spur gears

  1. #1
    Project Engineer
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    Crown and spur gears

    I have to make a small reversing gear assembly and i've proposed it to use 2 spur gears and a crown face gear. Now I have to do the models but i'm not finding much information of the modeling of a crown gear and how to make the teeth? Am I making it too complicated? I have it in solidworks now but it doesn't look right.

    Edit: Actually I know how to make teeth. but how do I determine the shape? I just took the tooth form from the spur gears and cut them across the crown shape.

  2. #2
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    Two spur gears might get ugly if the crown gear has a small diameter or a large tooth width. Can you use bevel gears? They handle the change in tooth form along the radial direction a bit better.

  3. #3
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    Quote Originally Posted by Hudson View Post
    Two spur gears might get ugly if the crown gear has a small diameter or a large tooth width. Can you use bevel gears? They handle the change in tooth form along the radial direction a bit better.
    Thanks for your reply. I'm just working out the bearings and shaft sizes now. here's a pic of the application. the crown gear is hollow in the middle so the teeth aren't very long.


    Attached Images Attached Images

  4. #4
    Principle Engineer
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    There is no 'good' shape for the crown gear. There will be point contact and sliding at best. That is the reason bevel gears exist.

    If this is a one off or low volume machine, why not consider using the guts from a small outboard motor gear box? You could employ two pinions on the drive shaft in the arrangement that you show and get better tooth contact. You might get the bearings in the deal.

    That suggestion made, you should know that most outboards have spiral bevel gears. Spiral bevels have a preferred direction that has separating forces that push the gears apart normally. You can imagine turning the bevel gears so that the pinion screws towards the gear center or away. It is possible to run them 'backwards' but it is not optimal. (Lots of folks backed the truck out of the driveway this morning)

    The old 5 hp Force 2 stroke made on the ancient Chrysler outboard tooling had straight cut gears and pinion if memory serves. (No reverse in that unit but you could buy two pinions.) The 5 h.p. Briggs outboard has spiral bevel gears with reverse. Higher hp outboards have reversing gear boxes

    You did not provide shaft speeds, power or other data so I'm just throwing out ideas.

  5. #5
    Technical Fellow jboggs's Avatar
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    The gears with perpendicular axes MUST be of the bevel type. There are configuration options within the bevel gear category (straight, spiral, etc.), but they must be beveled to work. That is the only way they will not bind up. Think of two cones rolling against each other sharing a common vertex.

    Pull out your old engineering texts and study the presentations on right-angle gearing arrangements. You will see that they all have some kind of conical feature to them.

    Picture an individual tooth on the gear you are calling the crown gear. As it travels through the contact area, its line of contact will be parallel to the horizontal plane at only one point. Now look at the pinion gear. Because its teeth are straight (not beveled) the line of contact on each tooth will remain horizontal all the way through the area of contact. Thus the binding.

    I would also suggest that if there is any possible way to do so you should use commercially available gears, or at least gears made by a company specializing in custom gears. This is an area where you want the experts to do it.

  6. #6
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    Quote Originally Posted by Hudson View Post
    There is no 'good' shape for the crown gear. There will be point contact and sliding at best. That is the reason bevel gears exist.

    If this is a one off or low volume machine, why not consider using the guts from a small outboard motor gear box? You could employ two pinions on the drive shaft in the arrangement that you show and get better tooth contact. You might get the bearings in the deal.

    That suggestion made, you should know that most outboards have spiral bevel gears. Spiral bevels have a preferred direction that has separating forces that push the gears apart normally. You can imagine turning the bevel gears so that the pinion screws towards the gear center or away. It is possible to run them 'backwards' but it is not optimal. (Lots of folks backed the truck out of the driveway this morning)

    The old 5 hp Force 2 stroke made on the ancient Chrysler outboard tooling had straight cut gears and pinion if memory serves. (No reverse in that unit but you could buy two pinions.) The 5 h.p. Briggs outboard has spiral bevel gears with reverse. Higher hp outboards have reversing gear boxes

    You did not provide shaft speeds, power or other data so I'm just throwing out ideas.
    Thank you for your reply and suggestion of the outboard motor drive as an example. Let me explain the operation I'm envisioning here.

    The motor that drives this assembly will be around 70 watt and the RPM by the time it gets to this arrangement will be about 20RPM and only turn one direction. It maybe doesn't show it in the picture I attached (Sorry) but only one of the spur gears will be engaged with the crown gear at a time, The shaft with those gears fixed to it will slide to have either one or the other gear engaged to provide the reversing function.

    When this unit is switched from forward to reverse it's stopped (the gears aren't driving) but there is a light load when switched back from the clockwise direction to the counterclockwise direction and I chose this concept because the two little spur gears can have their teeth inline (indexed), in fact the last iteration has these 2 gears as one long gear that can span both sides of the crown gear and slide between forward and reverse without the crown gear being allowed to freewheel in the middle of shifting. The teeth on all 3 of these gears will be engaged at the same time mid shift.

    The bevel gear arrangement is perfect if you have all the gears engaged and use a clutch to switch the power from one bevel gear or the other to the shaft.

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