Well this turned out to be a two part post, instead of one. It all started with me venturing into SolidWorks MotionManager, which is used to create little animations of assemblies as they are exploded and/or collapsed, rotated, dragged around, or just made to have their components move and interact with each other in the way they would do in real life. I’m working on a little video about it, using this assembly of an arbor press.

That’s the subject of my next post, but for now I wanted to tell you about the spur gear and the rack that form part of this assembly, if only because this is something I hadn’t tried before, because it required a bit of extra effort, and because displaying this information here might benefit someone else that is trying to do the same.

First of all, let me tell you where that spur gear and rack came from. I would love to say that I modeled them myself, but that would be a lie. No, I used some handy Toolbox parts, instead. If you’ve never used the Toolbox, maybe you should give it a try. Granted, you can’t really use the models of gears generated this way for manufacturing purposes, but they are at the very least a decent representation for when you need to simulate movement, like in the case of my arbor press assembly. So, if you have Toolbox with your seat of SolidWorks, go to the folder marked as Power Transmission, select Gears, right click on Spur Gear and then select Create Part from the menu.

A new part will open and you’ll see a model of a spur gear on the graphics area. Don’t despair if that’s not the spur gear you want, just wait for the property manager that will open to the left.

In this property manager, you can enter information about the gear you wish to generate, such as:

·         Diametral Pitch, which is the ratio equal to the number of teeth on a gear per inch of pitch diameter. The Pitch Diameter is the diameter of an imaginary pitch circle on which a gear tooth is designed. Pitch circles of two spur gears are supposed to be tangent for the gears to work. In the case of the spur gear and rack, the gear’s pitch circle must be tangent to the rack’s pitch line, which is the construction line in the middle of the tooth cut.

·         Number of teeth

·         Pressure Angle, which is the angle of direction of pressure between contacting teeth. It determines the size of the base circle and the shape of the involute teeth. It is common for the pressure angle to be 20 or 14 1/2 degrees.

·         Face Width, that’s the width of your gear.

·         Hub style and shaft diameter

·         Number of teeth to show

Once you enter this information, SolidWorks takes care of the rest and generates a model of your gear. You can then save it with a different name and add other features to it, like I did. Like I said before, this is not the proper way to design a gear, but only a way to obtain a decent representation of it.

Generating the rack involves a very similar process. From the same Gears folder, right click on Rack (spur rectangular) and select Create Part. You’ll see a similar property manager open to the left of the graphics area.

Once again, you are asked to provide values for the Diametral Pitch, Pressure Angle and Face Width. Diametral Pitch and Pressure Angle values should be the same ones you provided for the Spur Gear. You will also be asked to provide a value for the Pitch Height; that’s the distance measured from the pitch line to the bottom face of the rack. Once you have all your values ready, SolidWorks will generate a model of the rack for you. This rack can be used with the spur gear generated previously to simulate rack and pinion motion, with the use of a special mechanical mate, called, umm, the rack and pinion mate.

The rack and pinion mate is used for relating power transmission involving rotation and translation.  This can apply to a spur gear and a rack, or even to a wheel rolling on a surface. In this image you can see the case of a “wheel” and surface. In this case the wheel is resting on the surface of a little track and, besides other mates to prevent the wheel and track from wandering all over the graphics area, I applied a tangent mate between the circular surface of the wheel and the top surface of the track, just to make sure the wheel remains in touch with it as it rotates. Then, from the mechanical mates, I applied a RackPinion mate to the wheel and the track. For the rack definition, I chose an edge at the bottom of the flat track, and for Pinion/Gear definition, I used the edge of the wheel, as you can see in the image.  SolidWorks calculates what would be the pinion pitch diameter, which in this case is simply the diameter of the wheel. As a result of applying this mate, every time I rotate the wheel to the right, the track will slide to the left and vice versa. Notice, however, that in general, the use of this mate doesn’t even require that the two components come in contact with each other. For instance, if instead of applying the tangent mate between the surfaces I had applied a distance mate, leaving a considerable gap between them, the track would still slide to the left whenever the wheel rotates to the right. In the case of gears and racks, it means that simply applying this mate won’t provide the look of accurate motion; you have to do a couple other things to make sure it looks realistic.

First of all, if you look among the features listed for the spur gear, you’ll find a cut extrude feature named ToothCut, if you expand it, you’ll see a sketch by the name of TooCutSke; that’s the sketch of the teeth and pitch circle, so make sure it’s showing in the assembly (right click on it and select show). In the case of the rack, the sketch you are looking for is under ToothCutSim, by the name TooCutSkeSim. Make sure this sketch is also showing.

Before applying the RackPinion mate, locate the pitch circle in the gear’s sketch, as well as the pitch line in the rack’s sketch, and apply a distance mate between the center of the spur gear and the pitch line of the rack. The distance value must be the radius of the pitch circle. This is to make sure you have good tangency between both components. As I said before, with the RackPinion mate alone, you could have the spur gear and rack 20 feet away from each other, or vice versa, completely interfering with each other, and they would still move about, but that wouldn’t look very realistic, now would it? Another thing you need to do before applying the RackPinion mate is make sure the teeth or the spur gear and rack are meshing properly, that is that they are not interfering with each other. Adjust them if they are.

Now you are ready to apply the RackPinion mate. Under Rack definition, select any linear edge of the rack in the direction of travel. I used the lower edge of my rack for that purpose. In the Pinion/Gear definition, select the gear’s pitch circle. The property manager updates immediately with the value of the pinion pitch diameter, which is the same diameter of the spur gear’s pitch circle. Click OK and test the mate. If the rack is moving in the wrong direction, then edit the mate (rick click on it and select Edit) and mark or clear the Reverse option, then test again.

Not bad, huh? And you’ll see it in action in my next post!