CIRCULATING COpy UNIVERSITY OF IlUNOIS IGRICULlURE LIBRARY ~GRtCULTURE LIBRARY TAN OEM' TRACTORS Wendell Bowers and B. J. Butler I r 7 ' _"..-1 Circular 829 UNIVERSITY OF ILLINOIS COLLEGE OF AGRICULTURE Extension Service in Agriculture and Home Economics
T ANDEM TRACTORS give you a means of obtaining additional power to pull a large plow or disk. When they are separated, you can use both for work in their normal power range. By combining tractors, you can often save enough labor to eliminate the need for extra hired labor in peak work seasons. But you need a great deal of skill and ingenuity to build and operate a tandem tractor unit successfully. And because tandem tractor units are large and awkward to handle, you must think constantly of safety in design, construction, and operation. Here are some advantages and disadvantages of tandem tractors, as well as some suggestions that may help you if you decide to construct your own outfit. Advantages 1. More power for one operator. For example, two tractors that usually pull one three-bottom plow can be combined to pull one sixbottom plow. If you already own two tractors of this size, you need to invest only the time and material to hook the two together. Of course, additional tandem units mean added capacity. 2. Labor saving. For each hour the tandem unit is used, you save one hour of labor. You may be able to eliminate seasonal hired labor, which is often hard to get. Where large acreages are involved, eliminating hired labor may represent quite a financial saving. 3. Low cost per unit of power. If you tandemize existing tractors or buy additional used tractors, your total cost is quite low compared to buying an equal amount of new power in a single tractor. You can also take advantage of older tractors on your farm that are not ordinarily used for plowing and disking. 4. Flexibility. When you are not using them in tandem, you still have smaller tractors available for light and medium loads. Disadvantages 1. Awkwardness. You must be much more alert and skillful to operate a tandem unit than a single tractor because you have a dual set of controls. There is constant danger that the outfit will jackknife or overturn, or that you will lose control and then tend to panic. 2
T~0{ 1D. r 2 q p. ') You must take every possible precaution to insure your own safety and that of others when you construct your tandem outfit. 2. Arriving at a satisfactory operational unit difficult. Devising a single set of plans that will work well for all kinds of tractors is virtually impossible. Every operator has to work out his own plans since very few tandem tractor outfits are alike. 3. Implements to match tandem tractor power expensive - may not be readily available in proper size. If you do not have the proper implements to use with a tandem outfit, you run the risk of overpowering those you do have and making inefficient use of fuel. When you use tandems, you must start a planned, long range buying program for large tools. 4. Danger to transmission and final drive. Due to additional weight and tractive efficiency, there is an increased load on transmissions and rear axles. This higher load can cause greater expense and more loss of time than is usual with single tractors. 5. More turning room. Tandem tractors require more room for turning at the ends and in general more room for maneuvering, parking, and so on. Three ways to hook tractors in tandem In the spring of 1960, Ray L. Buhrmester, a senior student in the College of Agriculture, investigated the ways being used to hook tractors in tandem. He found three different methods being used. Of these, the one in which the front wheels were removed from the trailing tractor only was by far the most popular (Fig. 1). The other two methods (Figs. 2 and 3) made up a very small portion of the total number. Some farmers left the front wheels on both tractors and some removed the front wheels from both. For details on how these hook-ups are sometimes made, see Figs. 4, 5, and 6. Examination of the reports from farmers who had tandemized showed that selecting members for supporting the rear tractor created some difficulty. In one case, a 4-inch "I" beam was permanently deflected because it was not strong enough. Those who used 6-inch "I" beams or members with equivalent strength reported no trouble. Naturally, the range in costs of constructing the outfits 3
Tandem tractors with front wheels of the rear tractor removed. The drawbar of the front tractor should be reinforced to carry the extra load. (Fig. 1) Tandem tractors with front wheels removed from both tractors. The outfit is steered by hydraulic cylinders. This system is expensive and it is difficult to design a satisfactory unit. (Fig. 2) An inexpensive way to tandemize by leaving the front wheels on both tractors. The main problem is clearance under the front wheels of the rear tractor. (Fig. 3)
A closeup view of the connection used when the f ront w heels of the rear tractor were removed. (Fig. 4) A closeup view of the connection used with the front wheels removed from both tractors. (Fig. 5) A closeup view of the connection used with the front wheels on both tractors. (Fig. 6) 5
was wide. The figures below were determined from Buhrmester's survey and show the average time to hook the tractors together and to separate them for individual use, and the cost of tandemizing. Front wheels removed from- R ear Neither Both tractor tractor tractors Average time required to hook tractors together............. 50 minutes 25 minutes 3 hours Average time required to separate tractors for individual use....... 50 minutes 30 minutes 2Yz hours Average cost, excluding labor... $65 $45 $1300 Performance, cost, and practicality From the standpoint of performance, cost, and practicality, there is little question that it is best to tandemize by removing the front wheels of only the rear tractor. Those who did not remove the front wheels of either tractor saved time and money, but reported some difficulty with clearance under the front wheels of the rear tractor. R emoving the front wheels of both tractors presents engineering problems and is quite expensive. Working out a satisfactory method of steering is particularly difficult. A hydraulic system must be used and this gets to be quite complicated. A major factor, other than cost, is overloading the transmission and final drive of the front tractor. The stresses involved may exceed design limits of the manufacturers of the tractors, particularly when the tractors are pulling heavy loads. If you overload a tandem outfit of this type, you can expect high repair bills. A popular means for tandemizing tractors is to use the most modern tractor in front. For the second tractor, a seldom-used older tractor or a second-hand tractor is adapted. An example of this use of an older tractor is shown in Fig. 7. In any case, the tractors should be in a good state of repair at the time they are hooked together. The time to hook up the tractors in tandem and to separate them for individual use as the occasion demands is a major factor you must consider. Controls No matter which method of tandemizing you choose, you must provide some way to control both tractors. When you plan and 10 6
An older tractor may be used for the rear unit and left idle when the front tractor alone is needed. (Fig. 7) This is one example of a satisfactory tandem outfit. The rear tractor is well supported with a reinforced 6-inch "I" beam. The controls are well organized and readily accessible from the seat of front tractor. A (top) is the clutch; B (middle) is the hydraulic lever; and C (bottom) is the throttle control. For most hand clutches, a mechanical linkage is satisfactory and less complicated than a hydraulic control. (Fig. 8)
cate these controls, you must have a high regard for your own safety and that of others. Locate them so that they can be systemized and so that their operation is as convenient as you can possibly make them. Controls similar in form and located on a panel for convenience of operation are best (see Fig. 8 ). Ready access to these controls and their ease of operation cannot be overemphasized. You will need to have or provide controls for ( 1) steering; (2 ) throttle; (3) clutch; (4) ignition switches on gasoline or propane tractors and fuel shutoff for diesels; (5) hydraulic systems; and (6 ) emergency cut-off on the side of rear tractor to prevent jackknifing. Let's take a look at these one at a time and see what can be done to arrive at a satisfactory working unit. Steering. If you steer with the front wheels of the lead tractor, you have no particular problem. Most operators prefer to ride the front tractor and therefore need only the original steering apparatus. However, it is advisable for you to check the steering system thoroughly to make sure it is in a good state of repair. On the four-wheel units with both sets of front wheels removed, the rear wheels of the front tractor are used for steering. As has been pointed out earlier, steering this unit involves a hydraulic system and linkage that present a sizable engineering problem and one that has not been perfectly solved. Clutch. Where a tractor with a hand clutch is used for the rear unit, most operators prefer to operate it mechanically. If a footclutch tractor is used for the rear unit, a hydraulic control on the clutch is desirable. Those who have used two-way hydraulic cylinders on hand clutches report considerable trouble maintaining precise adjustment for operating the over-center action and for continually compensating for wear in the linkage. If a tractor with a foot clutch is used for the trailing tractor, installing a single action hydraulic cylinder (Fig. 9) is not too difficult. With a single action hydraulic cylinder and helper spring, displacement can be regulated to obtain the proper amount of travel. The use of a double action cylinder is unnecessary since foot clutches return to the engaged position when released. The high amount of pressure required to operate a foot clutch makes it difficult to devise a satisfactory linkage to operate by hand from the front tractor. 8
Foot clutches need a one-way hydraulic cylinder for disengagement. Spring pressure returns pedal when hydraulic pressure is released. (Fig. 9) Throttle of unmanned tractor can be adjusted by a mechanical linkage. There should be a means of holding it in position once it is regulated. (Fig. 10) 9
Throttle. The throttle control presents no problem. It is usually a rod extending from the throttle lever on the unmanned tractor to some location convenient to the operator on the manned tractor (Fig. 10). It is necessary initially to adjust the throttle to synchronize the speeds of the two tractors. In order to maintain the proper throttle setting, it is desirable to have a control that will hold its position and will not move or jiggle loose accidentally. Switches. An ignition switch for gasoline or propane tractors or fuel shut-off for diesels should be provided from the unmanned tractor to a spot near the manned one that is convenient to the operator. In any event, there should be a means for instantly shutting off the unmanned tractor in case the manned one should stall or become inoperable. In addition, there should be an ignition switch or fuel shut-off provided on the front of the rear tractor so the tractor will stall instantly in case it jackknifes. Figure 11 shows one way to install this type of switch. Hydraulic systems. Modern tractors hooked in tandem usually have ample facilities from the standpoint of hydraulic systems. But if you plan to tandemize older tractors, this is an important item to consider. Make sure that enough hydraulic controls will be available to take care of your equipment needs. The hydraulic system of the unmanned tractor could be used by extending controls so they are available from the manned tractor. Precautions for operating tandem tractors According to those with experience, operating tandem tractors is not child's play. In fact, before operating one it is considered a good idea to talk to some one who has. He may even take time to demonstrate some of the important factors in their operation. Be particlariy alert at all times for jackknifing. Jackknifing is usually caused by one of two things. Either a turn is too short and the rear tractor drives ahead while the front one is slowed down, or the front tractor stalls. The chances for jackknifing during a turn can be reduced by slowing the speed of the rear tractor first and letting the front tractor do the pulling. The quick shut-off device for the unmanned tractor is absolutely necessary in case the manned tractor stalls. This control should be readily available at all times and in operating condition. 10
As a safety measure, a contact should be connected to the ignition system and located on the side so that the engine will shut off instantly when the rear wheels of the front tractor hit the contact. (Fig. 11) 11
L MAKE SAFETY A HABIT when you operate tandem tractors. They can be dangerous if not handled properly, but can be operated in complete safety with a few common sense precautions. Stay well within practical limits of speed when turning and don't try to "show off" or take any unnecessary chances. Check the tractors and their controls every day. Check all shutoff devices to be sure they operate when you need them. Check the tires. Check the steering mechanism. Be on the watch for beginning failure of structural members. Practice with your tandem outfit before using it for field work. Take it out to a pasture or an open field where you will have plenty of room to turn. Find out how much distance you need at the end of a field to make a safe and satisfactory turn. Practice with your outfit until operating it is as automatic as driving a car. Prepared by Wendell Bowers and B. J. Butler, Assistant Professors of Agricultural Engineering, based partly on work by R. L. Buhrmester, student. Urbana, Illinois February, 1961 Cooperative Extension Work in Agriculture and Home Economics: University of Illinois, College of Agriculture, and the United States Department of Agriculture cooperating. LOUIS B. HOWARD, Director. Acts approved by Congress May 8 and June 30, 1914. 8M- 2-61- 73268