Universal joints must be the same size and be phased correctly

Tuesday, August 5, 2014

Present-day universal joints have a long life potential if they are oriented correctly and get a little grease as required

by RALPH WINFIELD

Most of us who started using driveshafts and power take off (pto) shafts many years ago do not give the universal joints much thought – unless they fail.

Many early tractors had their engine and transmission connected by a driveshaft, which incorporated two universal joints or a flexible coupling. Fast forward to the large articulating, four-wheel-drive tractors that started appearing in the 1960s. They have exactly the same type of connecting driveshaft, only much larger, and incorporate a slip joint that allows a length change as the tractor is turned (articulated).

The concept of using two universal joints to transmit power is not new. We have been using many pto shafts to transmit power from tractors to towed equipment, such as balers, combines and manure spreaders, for many years. With a little bit of care and some grease as required, they tend to be trouble-free power transmitters.

However, it is critical that, as an operator, you have a basic understanding of some of the sophisticated operational principles that are incorporated into the design of drivelines, so that they will provide that long trouble-free service life.

Two points of interest. First, the size of the two joints must be exactly the same. If not, vibration will occur when shaft alignment is not straight. Second, the yokes of the two universal joints must be in the correct alignment. Otherwise, the driveline will effectively self-destruct.

If you take a pto drive line assembly apart, you will note that the manufacturer has built-in provision(s) that you must put it back together with the correct yoke alignment. The rectangular (not square) sliding shaft or odd-shaped tube is not in place to make your task of putting the assemblies back together difficult. The two shaft parts must be put back together correctly to ensure that the two yokes, one on each end, are in the correct phase – that is, aligned with each other. (See the illustrations.)

Let me explain. When a driveshaft or pto shaft is turned or misaligned, as when you turn at the headlands, the driven and drive yokes actually reduce and exert the effective offset shaft lengths in such a way as to compensate the shaft angle and thus the relative joint speed to eliminate vibrations.

During my tenure as head of agricultural engineering at Ridgetown College, we encountered a set of misaligned pto shaft yokes on a new piece of farm equipment. Yes, it can happen. We have all heard about the Friday afternoon or Monday morning assembly blunders.

My technician at the college had replaced a set of universal joints on a drive shaft to a flex head corn picker. When he started up the repaired unit, he called and said, "Ralph, does that appear to be running correctly?" We soon determined that the yokes were out of phase. He capably turned one yoke 90 degrees by cutting and welding one of the shaft ends and the problem was solved.

Make no assumptions. Just because a piece of farm equipment or a pto shaft is new does not ensure that all universal joint yokes are phased correctly. Check for vibrations when the pto shaft is straight.

One must also use common sense. Universal joints are not intended to provide full 90-degree turns. The maximum off-alignment angle for each universal joint is 35 degrees. Therefore, the maximum total turn angle is 70 degrees.

Present-day universal joints have a long life potential if they are oriented correctly and the joints and/or the sliding shaft sections get a little bit of grease as required.

And finally, please do not confuse conventional universal joints with the newer constant velocity (CV) joints used in front-wheel-drive cars and to supply power to the mechanically driven front wheels (MFD) of many newer front-wheel-drive tractors that do not articulate. These joints must be protected from dust. All the car CV joints are equipped with protective flexible boots to keep the dust out and allow for lubrication. Many newer tractors with MFDs are also adequately protected and lubricated from the drive train. However, if you own or have just acquired an older tractor with MFDs, the joints must be greased regularly, primarily to expel dust. Please check the operator's manual for the procedure and frequency. BF

Agricultural engineer Ralph Winfield farms at Belmont in Elgin County.