The document describes the linear drive design for the HETDEX project. It includes two bridge actuators and one trolley actuator using SKF 60x10 precision roller screws. Unique design considerations include slow precise positioning for tracking and high-speed coarse positioning for slewing. Components like long roller screws are prone to issues like sagging. Analysis was done to address concerns like rotordynamic instability and stresses. Low temperature testing showed performance degradation which informed design changes like increased drive capacity. Technical collaboration involved various engineers and manufacturers to meet requirements.

1. HETDEX Linear Drive DesignTim Beets, PEUniversity of TexasCenter for Electromechanicst.beets@cem.utexas.edu(512) 232-4285

2. Tracker Linear Actuators2 Bridge actuators (X)SKF 60x10 precision roller screw w/preloaded nut18,800kg Payload4 meters travel1 Trolley actuator (Y)SKF 60x10 precision roller screw w/non-preloaded nut35° Inclination9,000kg Payload4 meters travelBridge and trolley ride THK linear bearing railsUpper X DriveY DriveTrolleyLower X DriveBridge

3. Unique Design ConsiderationsOperation:1) Slow-speed, precise positioning (tracking) and 2) high-speed, coarse positioning (slewing)Low temperatures atypical for roller screw applicationsComponents:Long roller screws are prone to rotordynamic instabilities and tend to sag along unsupported lengthHET structure is flexible with limited alignment capabilities and presents transient alignment issues under dynamic loading (i.e., tracker traverse)COTS drive componentsUpper X drive model and analysis of screw sag

4. Roller Screw ConcernsRotordynamic analysis predicted screw behaviorTracking speed is well below critical speedScrew is not rotated during slewing (high-speed traverses) and nut speed is reduced at center of travelSag of screw adds considerable load to nutAnalyses predicted unacceptable stresses at screw ends when nut was aligned at screw centerA single d.o.f. was added to allow the nut to pivot while traversing the sag regionResulting stresses are much lower—increased component life, controller consistencySlew drive pivot bearings

5. Low-Temperature TrialsTemperature effects cause large variation in drive performanceCurrent requirements doubled for velocity-controlled runsRandom behavior such as velocity spikes were observed at lower temperatureTest results drove decision to increase drive torque capacity to achieve performance targets at lower temperatures

6. Subsequent discussion with SKF resulted in changing grease in roller screw and slew drive bearings