Universal Rail Joint

There are several libraries that provide elemental components such as common multibody joints, mathematical blocks, functions and electrical components. These, in my opinion, can be one of the most useful types of libraries in any application. This is because they are not designed for any particular application; they are designed to recreate a particular effect or component.

One of my most common areas of development is multibody systems, where creating particular motions is the aim. This is mainly suspension systems for example wishbones, control links, springs and dampers. These can be created using some simple revolute, spherical and universal joints, translations and bodies from the Modelica Standard Library (MSL).

While systems like this are essential in development, there is a limit to their efficiency and ability characteristics. That’s where specialist libraries, such as the Claytex – Suspensions library, take over in efficiency, ease of use and ability.

One of the joints that doesn’t exist in MSL is a rail joint, where a frame is constrained to a specific path and angle. An easy example to imagine is a rollercoaster. There is no way, that I know of, to use a series of simple joints follow a path. So when a need for one arose we created one and added it to our Modelica Library, the Claytex library.

The rail joint is designed to act very predictably and simply, where it moves along a spline path without any friction. It is very similar to the prismatic joint, where one frame is free to move along the selected axis relative to the other frame; but in the rail joint, it only is free to move along the path.

The path itself can be as long as required and move in any axis, allowing any path to be followed as shown below. It is created by defining a spline as a set of points in X, Y and Z relative to the mount frame. The angle can also be specified at the same point allowing full control at an increment depicted by the input.

Figure: A 3D Wiggly Rail
Figure: A 3D Wiggly Rail

In addition of choosing the path of the joint, there is also the ability to control the angle of the joint independently to the motion. In most cases the angle of the joint is linked to the direction of travel of the joint. Either way the orientation is constrained, the torque can cause motion as in reality. For example, if a rail forms a complete circle applying torque around the same plane, it would cause the joint to move as demonstrated below:

Video 1: Circular rail being moved by torque, not force.

In many cases the rail joint is required to form a complete loop where the end of the rail runs into the start again, as shown below with an infinite loop. But there are also use cases where a single spline is followed with a start and an end, for example a car door slider.

Video 2: Infinite loop rail

The Claytex library is included as part of our solution packages. If you have any questions or you are interested in using this for an application please get in touch.

Written by: David Briant – Project Engineer

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