I was actually planning on posting about friction coefficients and the way it all works using an example of a simulation of these tong grabs, but before I do that, I thought I should share with you about a little problem I ran into while trying to use a special option available for the hinge mate and a way to work around it.

OK, first of all, let me tell you a bit about what these tong grabs do. Tong grabs or friction tongs are very clever contraptions that are often used to lift heavy, bulky loads.  The principle behind them is very simple:  as you pull them up, the tongs close on the sides of the object you wish to lift, if the friction coefficient between the object and the surface of the tongs that comes in contact with it (usually known as pads) is big enough, then the tongs will be able to lift the object.  Mine is a very simplified model, just for illustration purposes. There are several different kinds of tong grabs available, depending on the kind of load you wish to lift. I found some inspiration in the ones available from Bushman Equipment.

If you check the ones available from that site or some other, you’ll see that some of them have adjustable pads that come in touch with the load and that rotate a few degrees in order to accommodate wider or narrower loads. These pads are also often furnished with rubber, belting, or other materials to increase the friction coefficient as needed. I wanted to add some very simple rotating pads to my model, but I also wanted to limit their rotation to only a few degrees, to ensure they would not rotate to the opposite side or hang flat as the tongs were closing on the load.  For this purpose, it occurred to me that I could use a special kind of mate known as the Hinge Mate, and take advantage of the option “specify angle limit” available for this particular kind of mate.

The Hinge Mate is a mechanical mate that comes in pretty handy when running a motion study, since it’s usually recommended to add the kind of mates that will allow the model to behave as close to how it would do in real life as possible. In other words, if it behaves like a hinge, use a hinge mate. Think of how you usually mate the hinges of a door, for instance, you usually need two mates: concentric and coincident. The hinge mate combines those two mates in one and even offers the option of specifying a limit for the angle of rotation between the two components that form the “hinge”.

In the image, the hinge mate is being added between the grabs and the pad. The concentric selections are the cylindrical faces of the holes in the pad and the grabs. I’m not using any fasteners in this model, but if I was, this would be the hole for a bolt, pin or screw. The coincident selections are two faces that come in touch, shown in purple.

The option “specify angle limits” is being used here to limit the angle of rotation between the surfaces highlighted in pink.  The first value (50 degrees) is the current angle between the two surfaces, the second value is the maximum value that can ever be between those two surfaces and the third value is the minimum angle. This means that the pad will be able to rotate anywhere between the minimum and maximum angle as needed, depending on the width of the load.

 This is really cool, only problem was that when I tried it with my pads and ran the motion analysis, the “specify angle limit” option didn’t work at all. So, I asked the folks from SolidWorks if it was that this particular option wasn’t supported in SolidWorks Motion and they said to me that it is supposed to be supported, that this is not a bug, but a known issue still present in SolidWorks 2009 SP4.0, and they are working to fix it as I write this.  It may be fixed in a future service pack or release, but in the meantime, there is a way around it, and Mr. Matthew Derov, training specialist at DS SolidWorks, was really kind to explain to me how.

Here I share what he told me with you, in case you run into this same issue:

“The “Specify angle limits” option for hinge mates should be supported when using motion analysis.  This is actually a known issue and our developers are working on getting it fixed.  In the meantime, a work around for this issue does exist.  To specify an angle limit for your mate, de-select the “Specify angle limits” option in the hinge mate and set up a separate advanced “Angle” mate with a maximum and minimum angle.  I have attached a simple model for you to have a look at how I set it up (Motion Study 1). “

This is a screen shot of the model he attached. You can see that there are two mates between these parts: the hinge mate and the limit angle mate.

In this image you can appreciate the way he set up the hinge mate for the assembly.

“Please also note that a separate issue exists with the angle mate and motion analysis.  If your parts begin in perfect alignment (angle set to 0 degrees in attached model) there exists 2 separate solutions to the problem and the solver will not handle this properly.  Therefore, you must offset the starting angle slightly to define the angle direction.  This is also a known issue being worked on by our motion developers.  If you specify a slight offset (i.e. 1 deg) as done in my example, the solver will compute the solution correctly.”

This is a screen shot of the way he set up the limit angle mate in his example. Notice the small one degree offset that he’s talking about.  

The study he set up in Motion Analysis included only the force of gravity acting over the assembly, as you can see in the image.

One of the parts is fixed, while the other one can rotate within the range specified in the limit angle mate.  This part rotates by the effect of gravity, but stops when it reaches the value of the angle he specified earlier as the maximum limit.

Hope you find this information useful!