Table Velocity Errors

1 Problem

Occasionally, CT scanner owners have reported cradle velocity errors. This occurs while driving into the gantry, and with the cradle loaded down by a patient.

The most likely cause for the velocity error is an out-of-adjustment clutch on the cradle drive assembly. This clutch is adjusted to slip when a force of 36-39 pounds is exerted horizontally on the cradle while driving into the gantry. When the clutch slips, the velocity of the cradle will be far enough out of normal range to trigger an error, which stops the drive. Ideally, this would not occur within the normal operating range of less than 36 pounds. However, when the clutch is out of adjustment, it slips at lower drive forces that are within the normal range of operation. A one-direction roller-clutch, inside the clutch assembly, prevents any slipping when driving out of the gantry.

Although traction problems between the drive roller and cradle could exist, they are unlikely due to the rough bottom surface of the cradle, and due to the weight of the patient maintaining the contact between the cradle and roller. Another unlikely cause would be roller smoothness; the harder cradle surface is intentionally molded with a rough surface, which slightly distorts the roller's softer rubber surface, creating the high coefficient of friction. Generally, traction problems only occur when there is no patient weight to keep the cradle in contact with the roller. In this case, the shimming between the cradle and the carriage should be reviewed.

2 Solution

During the manufacturing of the clutch friction discs, a burr on the inside diameter of the disc (which relaxes after a period of time) was created, causing the clutch to go out of adjustment after leaving the factory. One of two courses of action can be followed, depending on the amount of time available for repair, availability of new parts, and availability of a force gauge:

• The existing clutch can be adjusted (see Cradle Clutch Adjustment procedure). This is the quickest procedure, since it does not require the cradle drive to be removed from the table. However, this is a two-person procedure, and requires a force gauge. Also, since the burrs have not been removed, the adjustment may not be maintained for a long period of time.

• The clutch can be disassembled, the burrs removed, and the clutch then reassembled and adjusted (see Cradle Clutch Repair procedure). This is the most time consuming procedure, but does not require a new clutch. However, this is a two-person procedure, and requires a force gauge and cradle drive removal.