As many of you do, we love to be a part of various forums out there and look at all the projects people are building. We think it’s awesome to see other ideas and tips. One particular forum we enjoy participating on is RCGroups. The other day we were perusing around and came across this cool DIY project from RCGroup user, Jackerbes (aka Jack) which would be a handy tool to have around- a motor and thrust testing stand.
The primary use of this is for any motors used to turn propellers like your airplanes, helicopters, and multicopters. The test stand allows hobbyists to mount a motor and prop and see the input power for the combo and how much thrust the propeller will generate with the motor. It also allows monitoring the motor for heat. Now for Jack’s project details re-posted from his blog…
The principal part is an inverted “U” shaped aluminum channel. That is mounted above two mini channels with beams that give it four pivot points. The mini channels contain the ends of the beams and keep the inverted “U” channel located and centered. The “U” shaped channel pivots on the four screws that hold the beams to the channel and the two mini channels and is pulled forwards under prop thrust. But there is virtually no visible movement at the motor because of the nature of the piezoelectric load cell in the electronic hanging scale. Nor is there any measurable drag from friction. I think it will provide accurate and repeatable results on forces as low as 10 grams or so.
The principal parts are the inverted “U” shaped channel mounted on four pivot points above the two mini channels. The “U” shaped channel is held in position slightly above two mini channels by four plastic beams and it pivots on the four screws that attach the beams to the “U” channel and the two mini channels. A 90 degree bracket has been added to the top of the “U” channel to act as a vertical firewall for mounting motors.
The pivot points allow the firewall and channel to be pulled forward from a vertical position by the pulling forces of the prop. With the beams attached and the nuts finger tight, the “U” channel has a total forward to backward travel of an inch or more but in use it is held with the beams vertical and restrained there by the bridle that attaches it to the hanging scale. And there is virtually no movement during testing.
The mini channels are attached to a base plate, a 5 Kilogram hanging scale with a LCD display lies on the base plate and the pulling forces are transferred to the scale by a short wire bridle. The bridle also keeps the beams in a vertical position.
I first assembled the beams and channels with the nuts just finger tight and experimented with the various hole locations to decide the holes to use for in the channels and beams. In the end I used the hole locations shown in images 03, 04, and 05 above. Then the stand was attached to a 4-1/2″ x 3/4″ x 15″ oak plank as seen here. Almost any material can be used for the base plate of course. The mini channels were attached even with the front end of the board with four servo mounting screws, one at each channel end. The channels are mounted 50mm apart (measured to the outsides of the channels) and the hole locations for the mounting screws holes marked through the holes in the mini channels. The screw holes are 40mm apart center to center.
This is a closeup of one of the pivot points on the front end of the test stand once the parts are mounted. Now do the final tightening on all of the nuts. If you use Nylock nuts take them down until they make contact and then back them off 1/4 turn to allow for free movement at the pivot points. If you use regular nuts back the nuts off about 1/4 turn and then use a toothpick and put a drop of blue Loctite on each nut at the nut/screw junction point.
The hanging loop and weighing hook were removed from an inexpensive 5 Kg hanging scale gotten from eBay. The bridle was made from a scrap of 10 AWG copper wire, almost any wire will work as long as it is not flexible. Attach the bridle, then the scale and determine the location for the “L” shaped screw that will restrain the hanging hook end of the scale. I used a small piece of 1/4″ sheet foam under the scale to rise it enough that the bridle was level.
I use the standard “X” mounts that are supplied with nearly all motors to mount motors to the firewall. The two “X” mounts I will use are modified a little for use with the test stand. The “X” mounts are two sizes, one has holes in a 16 x 19mm pattern for smaller motors and the other is slightly larger and has holes in a 19 x 25m pattern. The hole spacing on the mounting legs is 32mm on the small “X” mount and 42mm on the larger one. I used a countersink to allow the use of flat head machine screws for attaching the “X” mounts to the firewall. The larger mount is seen here and that is attached to the firewall with the two upper screws. I ran a #6-32 tap through the holes after countersinking and that enlarged the holes and tapped one two threads to retain the screws in the mount.
The mounts are attached to the firewall using pairs of the holes in the vertical firewall. The top two holes were enlarged a little (a Dremel tool or small rat tail file will work for that) and made slightly oblong to accommodate the 42mm center to center spacing of the screws on the larger “X” mount. The smaller “X” mounts used two of the other holes for mounting and in both cases the end of the shaft and Circlip are located in the large center hole and free to rotate freely with the shaft (an important consideration as any binding there will slow the motor).
You can see the smaller “X” mount on the back of a 2814 motor in this image. I used rubber cement to glue two flat washers over the holes for the 3mm screws that will hold the “X” mount to the back of the motor. This was to get some clearance on the heads of the screws on this thinner mount and also to take up some of the length of the 3mm x 6mm screws that are typically supplied for mounting motors. That is another important consideration in that the 6mm long screws may enter the backplate deeply enough to make contact with the windings or terminations and let the smoke out of a motor.
The two flat washers on the smaller mount can be seen better here. The larger “X” mount is thicker and the washers were not needed on that mount.
Parts and hardware from almost any hardware store (also available from the Servo City pages). The small holes in the Actobotics parts are sized for #6 screws. Those screws are 0.138″ in diameter, the equivalent metric screw size would be M3.5 x 0.60 screws. MS = machine screws, these could be hex socket cap screws or slotted head, Phillips head, or Button Head screws.
Here is the final project!
For more details and test results visit Jack’s forum post.