as the other beams of the assembly 10 can be made of a suitable metallic material e g steel to support desired loads with an adequate safety margin the auxiliary beam 22 is an elongate beam that extends through the interior of the central beam 16 in the illustrated embodiment the auxiliary beam 22 is a straight rectangular tubular member made of a metallic material e g steel although in alternative embodiments the auxiliary beam 22 can have other shapes the auxiliary beam 22 has a pair of parallel rails 26 disposed on each side only one pair of rails 26 is visible in fig1 to engage the rollers of the roller assemblies 24 on the central beam 16 the rails 26 can be machined into the auxiliary beam 22 first and second hoist assemblies 27 a and 27 b respectively are located at opposite ends of the auxiliary beam 22 the first and second hoist assemblies 27 a and 27 b are conventional cable or chain hoists that are capable of lifting at least about 272 kg 600 lbs with a suitable safety factor e g with a 5 u00d7 safety factor a screw type threaded drive shaft 28 is attached to the auxiliary beam 22 and the drive shaft 28 is driven by a motor assembly 30 mounted on the central beam 16 driving the motor assembly 30 induces movement of the auxiliary beam 22 via the drive shaft which allows horizontal longitudinal adjustment the motor assembly 30 can be a conventional electric motor with suitable gearing to engage the threads of the drive shaft 28 alternatively a chain drive or other suitable drive system can be used in further embodiments.