# Joint Initial Velocity: Cylindrical

Model ElementThe JointInitialvel_Cyl element defines the initial velocity for a cylindrical joint element. It allows you to specify the initial velocities for both the translational and the rotational degree of freedom.

## Format

<JointInitialvel_Cyl
joint_id  = "integer"
trans_iv  = "real"
rot_iv    = "real">
</JointInitialvel_Cyl>

## Attributes

joint_id
The ID of the cylindrical joint whose initial velocity is being specified.
trans_iv
The initial translation velocity of the cylindrical joint. The value specified is the translational velocity of the I marker of the joint relative to the J marker of the joint. The unit is model length units per model time unit.

A positive value implies a velocity along the z-axis of theJ Reference_Marker. A negative value implies a velocity along the negative z-axis.

rot_iv
The initial rotational velocity of the cylindrical joint. The value specified is the rotational velocity of the I marker of the joint relative to the J marker of the joint. The unit is radians per model time unit.
A positive value implies the angular velocity of Reference_Marker I as measured by Reference_Marker J in its coordinate system is in the counterclockwise direction.

## Example

Consider James Watt's flyball governor as shown schematically in the image below. The yellow circles depict hinges, the two red spheres depict the flyballs, and the blue body depicts the sleeve.

In his book, Out of Control, Kevin Kelly describes the flyball governor:

"Two leaden balls, each at the end of a stiff pendulum, swing from a pole. As the pole rotates, the balls spin out levitating higher the faster the system spins. Linkages scissored from the twirling pendulums slide up a sleeve on the pole, levering a valve which controls the speed of rotation by adjusting the steam."

The connection between the sleeve and the governor shaft may be modeled using a cylindrical joint element.

Assume that at the start of the simulation:
• The governor arms (and hence the sleeve) are spinning with angular velocity 3.147 radians per second.
• The sleeve is sliding upward on the shaft with velocity 4.5 mm/sec.

The initial velocity of the entire system may be specified simply by specifying the initial velocity of the cylindrical joint between the sleeve and the shaft, with id 1, as follows:

<JointInitialvel_Cyl
joint_id  = "1"
trans_iv  = "4.5"
rot_iv    = "3.147">
</JointInitialvel_Cyl>