# Universal joint physics simulation in unity

I‘m trying to create real physics simulation of the universal joint in unity. In my case universal joint is in vertical position. The whole mechanism consists of four objects: white ceiling, green shaft, blue universal joint and red shaft. All parts have Rigidbody component, with specific mass (ceiling – 1kg, green shaft – 0,05 kg, blue universal joint - 0,01 kg, red shaft – 0,05 kg). The mass of center of the parts are represented in the pictures. All objects dimensions are NOT scaled and represent the real dimensions of an object in cm. The Box Colliders or Mesh Colliders were not added to the objects. Green shaft center of mass point, Blue universal joint center of mass point, Red shaft center of mass point

The parameters for all objects are represented in the pictures: Green shaft parameters, Blue universal joint parameters and Red shaft parameters. To the ceiling object only Rigidbody component was added with an “Is Kinematic” option, without the “Use Gravity” option. One Fixed Joint and two Hinge Joints are used for objects. My aim is to create realistic physics behavior of this universal joint in simulation. For example, while playing the scene I want to be able to move the red shaft end, let go it and see how because of gravity and friction red shaft swings back to initial position. I want to do this with object components and project settings, not with a script if possible. If I succeed with universal joint physics I will later plan to add a point on the red shaft object end, which would have Constant Force component and would pull the red shaft end on demand with force to the certain direction. But the force which would pull the red shaft end would be based on real measurements.

The problem that I’m currently facing right now is with the object swinging back to initial position. If I leave Rigidbody and Hinge Joint parameters as it is represented in pictures and try to move red shaft end in scene play mode, the red shaft and blue joint swings back to initial position very very slowly. Like it would have no mass or would have very high friction. But if I turn on red shaft and blue Use Spring parameter in the Hinge Joint and set Spring value for example to 0.2 the shaft as expected goes back to initial position much faster. I also noticed that if I increase object Scale parameters (increase object size) for example from 1 to 100 it swings faster without the spring parameter turned on.

My first question which component or project parameters have the most impact on pulling object down in gravity direction?

Is where more friction friction parameters about which I should know while creating this universal joint model? Because so far I have changed only Drag, Angular Drag to 0, I have created a PhysicMaterial with almost no friction and added it to Default Material field in Project Settings->Physics. I have increased Default Solver Iterations and Default Solver Velocity Iterations almost to the maximum.

My third question would be, is it even possible to create a realistic universal joint shaft swinging without Spring option turned on in the Hinge Joint? Or should I better write the script that would define the red shaft swinging with gravity behavior?

First, Unity physics are not real physics, they are real-looking physics.

Second, Unity units are typically assumed to be in meters. When you set a position to <1, 0, 0> you should assume your object is at 1 meter in x.

Third, Unity physics uses colliders to determine volumes, for the purposes of moments of inertia.

So, when you say that

The mass of center of the parts are represented in the pictures. All objects dimensions are NOT scaled and represent the real dimensions of an object in cm. The Box Colliders or Mesh Colliders were not added to the objects.

It makes me think that (1) you are using the wrong scaling, like 1 = 1 centimeter instead of the assumed 1 = 1 meter, and (2) you're preventing Unity from being able to really run the physics calculations correctly because you're not providing volumetric information to the physics engine (which again is done via the collider volumes).

Also, your masses seem very, very, very small. You've got the universal joint at 10 grams, which is really not much at all.

You're seeing better results when you add a spring because the spring is adding a force, where your weights are small and your missing colliders are failing to do much with physics.

I would suggest adding mesh colliders and increasing the weights to get the behavior you want to see.