Frame Stiffness

Twin Column Vs. Double Channel

Competitor's ``Twin Column`` Design vs UNITED Double Channel Load Frame Construction

A testing machine load frame from a competing manufacturer uses columns & (round bars) of about the same size as the ball screws used in their testing machine. The stiffness or resistance to bending of those columns is about the same as that of the ball screws.

We ran a deflection test comparing the stiffness of a UNITED model STM testing machine column as used on our 5,000 lb to 20,000 lb capacity table top machines against a two inch diameter round steel bar.

The round bar was set up spanning two V blocks on 48 inch centers, with a dial indicator at the center to monitor deflection. Hector stood on the bar, the dial indicator showed deflection of 0.023 inch (see Photo 1).

Then the round bar was replaced with a model STM column channel, also on 48 inch centers and with the same dial indicator setup. Hector then stood on the column and the dial indicator showed deflection of 0.0005 inch. The round bar, under the same load, deflected 46 times as much as a United channel column (see Photo 2).

UNITED´S columns on our 120,000 pound machines are channels 9 by 9 inches, bent up from quarter inch thick cold rolled steel. This construction is more than 20 times stiffer than a three inch diameter round bar!

The UNITED crosshead is coupled to the columns by eight preloaded metal rollers which engage the inside corners of the two column channels. This construction easily meets the lateral motion limit of a competitor’s specification, (0.25 mm or 0.010 inch) over the full crosshead travel. In addition, the UNITED construction is much more resistant to lateral motion in the presence of side loads.

The above should settle questions about load frame stiffness and how well our crosshead guidance system controls lateral motion versus competing product which utilize ``Twin Columns``.