2011 21.3 Ask Albert
You mentioned that when I’m shopping for a tractor, I should have a PTO. Could you please tell me more about PTOs and what to look for in my search?
Dear PTO searcher,
A power take-off (PTO ) is a splined driveshaft, usually on a tractor or truck, which can be used to provide power to an attachment or separate machine. It is designed to connect and disconnect easily. The power take-off allows implements to draw energy from the tractor’s engine.
The PTO is engaged/disengaged using a clutch and a control mechanism which operates on the PTO itself. Typically a mechanical linkage is used to engage the PTO , but electric, hydraulic, or air valve mechanisms are also available.
The unit will be rated according to the continuous horsepower that can be applied through the PTO and different models will offer different PTO shaft rotation to engine RPM ratios. The principle aspect to look for when inspecting a PTO is that it is “live”. This refers to the way of actuating through the clutch. The old style of activating was interrupted where one pushed the clutch in which engages all the power transfer from the engine. Originally, when pushing the clutch in, everything stopped (i.e., transmission, no forward or backward motion of the vehicle). Later, two-stage clutching was developed, where one pushed 2/3rds of the way down on the clutch to disengage the transmission, then all the way to disengage the PTO.
In the ‘60s the independent PTO came about. The independent clutch for the PTO from the transmission stayed mainly the same except the method of engaging clutch was either manual, hydraulic or electric. It was a shaft tied directly to the engine crank, but the method of transferring power through the shaft was the same.
By contrast, “live” PTO s transfer power from the engine regardless of if the tractor is in gear or not. There is a separate clutch for the PTO that is totally independent and is usually an over center clutch that’s pulled by hand.
Nowadays most PTO s have a “clutch basket.” Clutch baskets have a friction disc on one side of the shaft attached to the crankshaft, on the other side are steel discs attached to a set of gears to reduce speed and change rotation that attaches to the PTO output shaft. Steel and friction compression occurs when the drive side spins free while engaged, friction overcomes the resistance, and the PTO shaft spins the attached equipment.
The hand-activated system has a distinct advantage of allowing the user to feel as the PTO clutch is being engaged. It is easier on the various moving parts and a smoother transition. Contrastly, electromagnetic engagement is either on or off, engaged by flipping a switch. This is rough on equipment, due to the intensity of when and how it is engaged.
Servicing The Machine
If the machine needs to be rotated for greasing, it can be best done with a manually engaged PTO . If the shaft is turned electrically, the operator looses control of how far and fast the machine is moved or revolved.
Be very careful when servicing, as the PTO and its associated shafts and universal joints are a common cause of incidents and injury in farming and industry. Some implements use plastic guards, but caution must be still be exercised around PTO shafts when they are plugged into a tractor.
Some machines are hydraulic driven and the PTO will connect directly to a hydraulic pump. A hydraulic clutch system is an internal hydraulic system in a tractor consisting of steering, brakes, clutch, and 3 point. Some John Deeres were constructed this way. Moving a small lever on these systems diverts hydraulic pressure that engages the PTO clutch.
There are larger horsepower tractors that have a direct to-engine shaft output to run hydraulic pumps and have the standard PTO available. Typical applications include rotovator, mower, chipper, pumps, sprayers, balers, running a water pump on a fire engine or water truck, powering a blower system, raising and lowering a dump truck bed, operating the mechanical arm on a bucket truck, operating a winch on a tow truck, and operating the compactor on a garbage truck.
Attaching The Shaft to the PTO
K.I.S.S. Keep it simple. Sometimes the tradeoff is that a mechanism is more manual than automatic, but there are fewer moving parts to break. Generally, there are 3 different styles of attaching the PTO :
- Push back the collar. Twist the 3 ball bearings, pull the collar back, put the shaft on, and release it so that the collar seats and grabs on.
- Cinch bolt. Put the bolt through and tighten it up.
- Push pin (spring loaded). Push the pin back, slide the shaft on and release the pin and the collar seats in.
Safety. Let the spinning shaft stop on it’s own. DON ’T grab one while it is going, as it will spin you. Make sure the shaft is securely attached. If it isn’t and you try to engage the PTO, it isn’t pretty and can be expensive – the splines can be bent and things could go flying. Listen for the collar to snap or click in to know that the shaft is securely connected. Give the shaft a tug and it should stay attached. If it comes disconnected, you probably didn’t get the connection securely done and need to redo it.
REVOLUTIONS PER MINUTE (RPM)
There are three general categories of dimension and speed.
- The original type calls for operation at 540 revolutions per minute (rpm). A shaft that rotates at 540 rpm has 6 splines on it, and a diameter of 1 3/8”.
- Newer types, supporting higher power applications, operate at 1000 rpm and have 21 splines of 1 3/8” diameter.
- Less often there is the third option of an engine speed PTO for use of hydraulic pumps or sprayer pumps. Some tractors have single-speed PTO s while some have a 2 speed in which you change the output shaft from a 6 to a 21 spline and shift a gear on the inside of the transmission, or some have 2 separate shafts out of the housing.
All 3 RPM types rotate clockwise when viewed from the rear of the tractor. It is customary for agricultural machine manufacturers to provide the nominal PTO power specification. The educated question to ask when tractor shopping is whether the PTO is 540 rpm or 1000 and how the PTO is engaged. Is it a single or dual speed PTO?
If you don’t need the versatility of the dual speed, stay with a single speed PTO . Look at the equipment and attachments you’re going to be running and let that guide your choices.
CONFERENCE EQUIPMENT WORKSHOP
If you’re planning to attend the Conference on November 11 – 13, 2011 in Yakima, submit questions for the equipment workshop. Email me at email@example.com
Yours building/repairing haying equipment in the shop,
Ask Albert! is a regular column by one of Tilth Producers most excellent mechanic/inventor/farmers. E-mail questions/comments to firstname.lastname@example.org or snail-mail: Ask Albert! c/o Tilth Producers, 4649 Sunnyside Ave N, Seattle, WA 98103.
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