# Posts tagged torsion

## Parmec ball joint and articulated pendulum example

- 31 July 2018

PARMEC includes its first algebraic joint – a ball joint – which is realised by nullifying relative velocity of a coincident point between two particles. BALL_JOINT command is used to define a pair of particles and the point. Since joints are realised as algebraic constraints they do not influence the stable time step. Instead a linear system is solved at every time step. Any number of joints can be added to the system, although linear dependence should be avoided (QR solver can be used in such case, see the Config.mak file). The articulated pendulum example demonstrates performance of the ball joint, also in conjunction with the torsional spring. The example is summarised below. (…)

## Parmec spherical joint

- 13 June 2018

Torsion spring has now been added to PARMEC, allowing to model angular hinges with rotational limits and spherical joints. TORSION_SPRING command is used to define an angle–torque relationship for three angles of rotation about user specified local coordinate axes (x–roll, y–pitch, z–yaw). The command includes a cone constraint which allows to model spherical joints in roll-pitch, roll-yaw, and pitch-yaw planes. Example application application of this functionality is included as tests/spherical_joint.py. A ParaView animation generated by this test is included below. (…)