Tractor Dynomometer


The dynomometer will work with PTO or belt.  We have run tractors with  7 1/4HP up to 74HP and PTO torque up to 1004 FT-LBS
It was built from the front wheel rotor off a heavy GMC 4WD pickup and 2 brake calipers.

The red can on top holds 3 cups of water to cool the brake. With over 25HP the water dripping onto the brake turns to steam as the test is run.

A load cell measures torque and a pickup provides 25 pulses per revolution to the custom designed PIC based microcomputer.  The computer displays RPM, Torque and calculates HP every secoond.  It also saves the highest torque and HP for each run. 

To operate, connect tractor, clear maximum readings from last run and press on maser cylinder lever to apply brake. 

	Joseph in Mississippi emailed me and asked for more details so he can build a copy of the dynomometer for himself. Here are some pictures. The front of the computer with the analog dial, 3 digit 7 segment display, power switch at the upper right and input buttons at left. Click on any image for a BIG view.
	Inside of the computer,  The PC board is from another project, it has a PIC microprocessor and 3 LED displays.  it has drivers for the stepper motor used for the analog display. Green wire mods the board to what I need here. The perf board is the interface for the load cell. The 555 timer makes -8v and the 2 voltage regulators are for +10v.  The LM324 is a quad amp that does a bridge amp. The ouptut of the amp feeds to the analog input of the PIC. The 2 pots adjust the gain and offset of the load cell amp.  The resistor, cap and transistor at the right of the perf board take the pulses from the stepper motor turned by the engine and make a nice signal for the computer. The wires to the connector are: from left to right; twisted signal from load cell, twisted power to load cell, +12V and Ground, and pulse input from the RPM stepper motor.  The wires to the computer are at the bottom of the perf board.
	Close up of the front.   The bolt, spacer and cup are removed for the 1 3/8" PTO shaft.  These spacers and bolt hold on the flat belt pulley.  The spline shaft is custom made to fit the center of the spindle and 1 3/8 spline. The "V" shaped angle couples the torque down to the short rod at the bottom. The force then is linked to the vertical arm and up to the load cell (not shown) Rotate your monitor clockwise 1/4 turn!
	Back (operators view) showing master cylinder at right, steel brake line with a coil to allow the brakes to turn and a "T" to feed both brake calipers. The 4" pipe is turned smooth and 4 wood bearings ride on it.  The frame for the brakes is free to turn.  It only turns 1 or 2 degrees when loaded. The small block of 2x4 is a base for the watter dripper.  It has a tube that feeds water into the center of the vent ribs in the rotor.
	Here you  can see the linkage to the load cell.  This link is the only thing that limits the brake from turning with the rotor. The arm lengths 24" from center of rotor to the lower link, 1" from the link on the second arm to the frame and 22" back up to the load cell or 528 to 1 ratio. 100 ft-lbs = 2.2 pounds of force on the cell.  Max about 1200 ft-lbs. The red thing is the 25# load cell. The 2 flat bars just below the load cell guide the bar and have a stop screw so it can't move away from the load cell. There you go turning your head the other way again.
	Up in side the center pipe you can see the stepper motor from a 5" floppy drive. It is mounted into the spline shaft and the slotted bracket at the top keeps it from turning. A bolt welded to the pipe is the stop. This puts out 25 pulses per revolution for the tach input. The shiny ring is the spindle bearing and you can see 2 of the 4 bolts that hold the spindle to the pipe.
	And the flat belt pulley from a 1927 Farmal (Regular) with a 1 3/8 spline hole that fits on the shaft. To the left you can see one end of the 75HP PTO shaft