Archive for Draft Depth
Tractor 3pt Hitch Draft Control Types
First we must determine the meaning of the word draft.
Draft = Depth
Draft = Pulling force
For the purpose of this article it is important to note that a tractors draft control is NOT a depth control system. It is a LOAD control system controlling the load placed on the tractor by an implement and it is set by the operator by the movement of the 3pt hitch hydraulic rockshaft control valve or thru mechanical or electrical movement of the rockshaft valve. Now we can see that this lever also controls/limits the depth an implement may go into the ground but this control of depth is affecting the vertical load placed on the rear of the tractor. As the depth of an implement increases so does the draft (pulling/vertical load) increase until horsepower or traction has reached its limit and the tractor stalls or spins.
For our John Deere tractors they use three types of draft control systems:
Select Control Type: This is the most basic system that John Deere uses to control the 3pt hitch system. This type uses a rockshaft control lever that has an up, down and neutral position. Implement positioning is controlled by how quickly an operator can return the lever to the neutral position. You push the rockshaft control lever forward to lower and back to raise an implement. The middle position is neutral and on newer tractors the control lever will return to neutral automatically as soon as the operator releases the control lever from up or down positions while some older tractors the operator had to return the lever to neutral manually. With this basic system an operator may get the implement in the same position 1 out of 100 times. This type is neither precise nor accurate.
Position Control Type: This type of control system involves a rockshaft control lever that has a full up and full down position as well as the ability of stopping at any point in-between. This JD type usually uses a scaled strip next to the control lever with a moveable stop so an operator may raise and lower the implement to the predetermined “stop” position. If you were looking to have your plow, box blade or mid-mount mower raise and lower by the 3pt hitch to the same position this would be the control type you would want to select. It should be mentioned that the precision and accuracy of the position control is not mentioned and this is due to lack of data but the precision appears to be relatively high.
Load Sensing Control Type: Load-sensing types are normally used in conjunction with the position control type and uses mechanical or electrical sensors to control the positioning of the 3pt arms as load forces increase or decrease. Load-sensing is found on John Deere tractors that are capable of larger ground engaging equipment, as the load-sensing was originally designed for plowing implements. When used with position control the load sensing controls will raise the 3pt arms when the draft increases and lowers them back to the position control set stop point when the load decreases.
Some historical types of load sensing used the top link of the 3pt hitch as the control point. The top link had a coil/leaf/torsion spring that would attach mechanically to the rockshaft control valve and allow for raising or lowering of the 3pt arms based on the load conditions placed on the top link. This mechanical-hydraulic system was very popular. Another variation of this same system mechanically attached to the lower arms verses the top link. This change was more prevalent when farmers were switching from single plows to multi-furrow plow setups. Still there were other older systems that monitored rear axle torque to raise and lower the 3pt arms and ones that used a separate wheel system attached mechanically to the rockshaft control valve to control the 3pt arms.
Newer load-sense control types use electro-hydraulic systems to control the movement of the 3pt arms. One type of the electro-mechanical systems uses electrical sensors the are inside the rockshaft cover or outside and connected to a mechanical lever that attaches to the lower 3pt arms and the control of the rockshaft control valve can be mechanically controlled or with electronic solenoids. Another electrical system uses all electronics to monitor the load on the tractor and move the 3pt arms. It does this with electrical sensors on the 3pt arms or rockshaft and sensors that monitor the speed of the tractor (with the aid of radar, GPS, or axle speed sensors) and either sensor input can cause the movement of the 3pt arms. The electronic systems are all controlled with the aid of an ECU that can monitor all the inputs and control the 3pt arms with no operator input once the system is set and activated.