Easy-to-Build Force Multiplying Thrust Stand

[larry cottrill]

This is a lot easier to build than it appears in the drawing, and should give very accurate and easy to read thrust measurements. There are two "secrets" to this design: The use of perforated square steel tubing for the entire structure; and the use of the fabulous Health-O-Meter(R) 330 lb bathroom scale, which features good internal damping and a SIX INCH diameter dial. With the stand built as shown, you will read pounds on the scale for tenths of pounds of thrust, i.e. 4.75 lb of thrust will show on the scale as 47.5 pounds because of the multiplying action. 1 lb increments are at around 3 or 4 mm centers around the dial, so you can easily tell whether you're on a mark or between two neighboring ones; so, reading to an accuracy of .05 lbF should be quite reasonable.

The leverage (in this case, 10-to-1) will be extremely accurate, because the perforated metal tube is manufactured in such a way that the punched holes are very accurately spaced (usually at 1-inch centers in tubing of reasonable size for this project). The main beams of the lever are plenty strong enough to handle the forces involved without bending, and a "tie beam" is provided to hold the thrust force vertical (and perfectly parallel to the application of the multiplied force to the scale tread). Instead of automatically choosing some nominal bolt size, you should select whatever bolts, English OR Meric, that best fit the holes without noticeable drag. A little "slop" won't mean much though, because the forces will all be vertical and so all the bolts will tend to self-center in the tops and bottoms of the holes. By removing two long bolts (the bolts aligning the "pressure posts" to the beams) a completely different ratio can be chosen.

There are only three welded parts: A pair of "main struts" and a small thrust-bearing frame that appears near the left side of the drawing. The thrust frame is shown in plan in End View A-A -- the "platform rails" welded across the top can be whatever works for you. The main strut plan is shown in Section B-B; each comprises a carefully cut section of the square tube, a couple of small pipe sections and a couple of heavy washers, all welded into a rigid unit. If the struts seem a bit light and spindly, keep in mind that they each have a very strong moment connection at the bottom end, and the thrust force places them in tension, NOT compression -- they should hang in there just fine.

The scale simply rests on the platform deck. The tread should be marked with an outline of the wooden "pressure foot" so it can be quickly set up under the beam assembly. The scale can be simply moved to a new position if the multiplying ratio is changed. The bottom of the pressure foot needs to be carved to a slight concave curvature to match the slightly domed form of the scale tread. Once the pressure foot is lowered into place on the scale, the vertical axes of the pressure posts and the thrust frame become stable. There should also be a cuple of bubble levels secured to the platform as shown in the drawing, since these axes will only be really vertical if the platform (and therefore, the scale) is well leveled. The internal damping on these scales is so good that there is almost no "bounce" at all when I step off ours -- it just heads for zero, overshoots about two little marks, and dies on zero.

A "counter beam" holding a moveable counterweight is provided so that nulling the scale to 0 will be possible. Basically, the weight is set up to approximately counterbalance the weight of the apparatus and the engine under test. Then, final nulling is done with the adjusting screw on the scale. Obviously, this should be done with the engine rigged exactly as it will be during running -- typically, the fuel line should be attached but the ignition wires and starting air hose (if any) should be absent. The weight can be made to take advantage of the perforations in the tube by using a drilled stud and one of those wire spring clips (I don't know what they're called -- see the right end of the Top View if you don't understand what I'm talking about).

This project also tests your carpentry (or maybe, cabinet-making) skills. Ha. The 3/4 inch plywood is probably going to be the most expensive part of the whole project, unless you happen to have some lying around in your shed or something. I would suggest glue and screws, or at least glue and nails, to tie the deck onto the 2x4 framing of the platform. And, you probably ought to give it a good coat of spar varnish before bolting any of the non-wood parts on. The two drilled and counterbored holes for the attachment of the main struts should be carefully done, and should fit the bolts so you have to tap them through with some light hammering. Do these holes with a drill press if possible, to get them perpendicular to the deck plane.

So, what do you think -- is this worth doing? All comments welcome.

L Cottrill