Fly System - Fly System Components - Arbors

Arbors

A counterweight arbor is a sturdy mechanical assembly that serves as a carriage for counterweights. In its simplest form, an arbor consists of two horizontal steel plates, a top plate and bottom plate, tied together by two vertical steel connecting rods. Counterweights are stacked as required on the arbor's bottom plate to balance the line set load, with the weights held in place by the connecting rods.

A flat tie bar at the rear of the arbor also connects the top and bottom plates. Guide shoes at the top and bottom of the tie bar guide the arbor along tracks mounted to the side stage wall. UHMWPE pads on the guide shoes limit friction between guide shoe and track as the arbor travels.

Spreader plates are thin steel plates with holes through which the arbor connecting rods pass. Spreader plates are lowered onto the counterweights in a distributed fashion as the counterweight stack is being built. Typically one spreader plate is placed on top of every two feet of counterweight in the stack. Finally, a locking plate is lowered onto the completed, interleaved stack of counterweights and spreader plates and secured in place with a thumbscrew.

Spreader plates serve to maintain consistent spacing between the arbor rods to ensure reliable containment of the counterweights under normal operating conditions. Also, in the event of a "runaway" (loss of control of an unbalanced lineset), the spreader plates will prevent the arbor rods from bending outward, and thus releasing the counterweights upon arbor impact at the end of its travel.

Counterweight arbors are commonly between 8 and 12 feet in length and can often support stacks of weights between 1500 and 2400 pounds, or beyond. In order to avoid unreasonably tall counterweight stacks at high capacity line sets, arbors may employ more than one counterweight stack. Such arbors use multiple-width top and bottom plates with a tie bar and pair of connecting rods provided at each counterweight stack.

Counterweight rigging systems use either tracked or wire-guided arbor guide systems. The tracks or wire guides limit lateral movement of the arbors during arbor travel. Wire-guided systems have lower capacities and are not in common use.

In addition to guiding the arbors, a tracked counterweight system is provided with bump stops at arbor high and low trim that establish the limits of an arbor's travel.

A tracked guide system is sometimes referred to as a T-bar wall, as the tracks are commonly made of steel T-sections. Aluminum arbor guide tracks are a relatively recent alternative, often using a J profile, instead of a T profile, to facilitate system installation.