Lesson 14. Design of various components of three point hitch systems.

The tractor hitch system is that essential element of the tractor which binds the prime mover and the implement into a single working unit. The tractor or the implement will not perform alone but must work together like two wheels of a cart. The word hitch is defined as a single articulated point or combination of articulated points and links through which the tractor delivers tractive effort in the form of full or push to counteract a draft force of an implement or draft producing body. And the hitch point (virtual or real) is the point on the tractor (as integral part or otherwise) through which the “line of push” must pass.

In the beginning of traction engine development, the prime movers and implements (mostly gang plows) were one integral unit. Later as the traction engine prime movers (tractors) became more popular, tractor and implements were made separate units. This was the point of introduction of hitch system in tractors and the hitch was a single-point hitch. The depth control for the single point hitch was internal to the implement. In late 1930’s Ferguson, an Englishman developed a hitch system wherein the implement could be integrally mounted to the tractor. The hitch system was called a “three point hitch”. The linkage was designed with a sensing system that could sense and control implement draft. The hitch system became so popular that now every tractor models has it.

1. Design Concept

The design of a machine system (hitch system) must be carried out in five stages. First, the purpose/functions of the system must be considered. Second, necessary motions of components of the system must be studied. Third, some devices must be selected which will produce the required motions. Fourth, the forces acting on the members must be analyzed and overload factors must be considered. Finally, a choice of materials must be made and the parts properly proportioned to withstand the maximum forces determined. In hitch systems, it is very important to understand the purpose/ functions in order to relate geometrical proportions as well as loads imposed for structural requirements. Effective designing, then entails disciplined methodology in the treatment and analysis of the information.

2. Hitch System Components

Three point hitch system is the only hitch system now available for all implements except trailers and combines. Thus, information will be restricted to three point hitch system. Three types of hitching possible with three point hitch system are free float (towed), semi-mounting, and fully mounting. The components of a three point hitch system and their functions along with numbers required are given in Table 14.1.


141.png
 
Fig. 14.1: Major component
s of a hitch system


Table 14.1: Major functions of various components of a hitch system

Component

No. required

Functions

1.

Upper Link

1

Basic component of hitch system. Connects implement to tractor. Transfers hitching forces of tractor to implement and of implement to tractor

2.

Lower Link

2

-do-

3.

Lateral Limiters

2

Limits the travel of implement in horizontal plane thereby inhibits the horizontal sway.

4.

Lift Link

2

Lifts the implements and supports the implement in transport position. It provides lateral stability and lateral levelling to the implement.

5.

Lift Arm

2

It provides lateral stability. It lifts implement by giving needed force and motion to lift link and supports implement in transport position.

6.

Rocking Arm

1

It connects cylinder and rockshaft. It converts linear motion of cylinder into rotation.

7.

Rockshaft

1

It transmits the rotating motion of rocking arm to lift arm. 

8.

Cylinder

1

It is source of power for lifting implement. It gives linear motion to rocking arm.

 


3. Design Requirements

There are a number of features which are required to be provided in a good mounted hitch so that the hitch system is able to meet all functional requirements. The following requirements of hitch system are self-explanatory:

1) Depth control of implement

2) Load transfer to drive wheels of two wheel drive tractor

3) Lateral sway and center-ability

4) Inter-changeability of implement and tractor

5) Fore and aft levelling of implement

6) Lateral levelling of implement

7) Limiting sway of implement

8) Locking hitch laterally when in transport or using PTO with implement or semi-mounted implement

9) Quick and easy attachment and detachment of implement

10) Adequate lifting capacity of the hydraulic system

11) Independent vertical float of each lower link hitch point

12)  Pitching the implement as it is raised

13)  Simple and easy adjustment of the hitch

4. Important Features

(i) Depth Control

A good mounted hitch system should be able to provide adequate depth control of the implement. The word “adequate” is used because there are two philosophies on depth control. One; the accurate control of  a preset tool depth is necessary at all times regardless of implement draft (position control) and, two: the tool depth vary in different soil-draft-load conditions to maintain constant draft load on tractor (draft control). The depth control in the later philosophy is achieved by sensing a force, which is proportional to the implement draft, by a variety of systems for upper or lower links. In either case, the basic function that must be accomplished by the sensing mechanism is to create a movement which is directly proportional to draft. The motion of sensing mechanism is then used by hydraulic system in such a way that implement is lifted if the draft is more than preset draft and implement is lowered if the draft is less than the preset draft. The sensing mechanism for depth is many a times a cam on rock-shaft. The motion of rock-shaft is then used by hydraulic system in such a way that the implement is lifted if the depth is more than the preset depth and the implement is lowered if the depth is less than the preset depth. Regulation of the sensing mechanism is another important aspect which requires attention of the designers. It is given by the following formulae:

ii. Draft Control
 
142.png... (14.1)
                       
Where:             AFSR=Average force to start hitch to raise

                        AFSL=Average force to start hitch to lower

iii. Position Control
 
  143.png               ... (14.2)
                                   
Where:

ADSR=Average displacement of sensor from zero depth position to start hitch to raise

ADSL=Average displacement of sensor from zero depth position to start hitch to lower

Poor regulation occurs due to friction, tolerances and clearances which must be reduced to a bare minimum. A typical regulation curve is shown in Fig.14.2.

 

144.png
Fig. 14.2: Regulation curve for draft control by sensing force in the lower link

5. Dynamic Load Transfer

A good mounted implement should produce maximum possible amount of load transfer to tractor drive wheels. The free body diagram of tractor is shown in Fig. 14.3. The load transfer moment and dynamic load transfer is calculated as given below:
 
145.png                                                ... (14.3)
 
Where:             PV      = Pull in the vertical plane (resultant of L and V)

                       B         = Angle of pull in vertical

                      RRS      = Reaction on rear wheel under static condition and Y and Z are as shown in Fig. 14.3.

For rear wheel drive tractor, the dynamic load transfer (DLT) on to drive wheels is given below:
                       
            146.png                  ... (14.4)
                                                           
Total load increment on drive wheels is sum of dynamic load transfer and vertical component of the pull.
 
             147.png      ... (14.5)                                                                      

148.png

Fig. 14.3: Free body diagram for dynamic load transfer


Thus total load transfer is function of the magnitude of PV, the distance Y, angle of pull B and shift in contact point Z. Increasing Z and PV increases total dynamic load transfer. Increasing wheelbase reduces the total dynamic load transfer. The distance Y is given by the following equation:
 
  149.png                     ... (14.6)
 

Where:        

XI =    Horizontal distance (from rear axle) of the point of intersection of the result out of useful soil forces, parasitic soil forces and weight of implement with parasitic soil forces.

YI =    Vertical distance (below the contact point of rear wheels) of intersection of the resultant of useful soil forces, parasitic soil forces and weight of implement with parasitic soil forces.

The parasitic soil forces are zero for mounted implement and maximum for pull type implement. Hence, for mounted implements, XI and YI are constant.

The value of B lies between 0 and 90 degrees. It can be easily proved that the TLT is positively increasing function w.r.t. angle B by substituting the value of Y from equation 14.6 into equation 14.5. The first derivative of TLT w.r.t. B is always positive if B lies between 0 and90 degrees. Hence hitch geometry in vertical plane affects dynamic weight transfer.

6. Center-ability (Lateral stiffness)

The term center-ability refers to the tendency of the hitch system to recenter itself when the hitch system becomes decentered (Fig.14.4). The center-ability is affected by the hitch geometry in the horizontal plane. The center-ability is provided by center-ability force of an implement. This force can be readily obtained from the measurement of a restoring force as the implement is forced sideways. The center-ability is also referred to the force tending to recenter a laterally displaced hitch. The center-ability force of a three point hitch can be increased by making several changes in the horizontal geometry of a hitch system as listed below:

1. Increase convergence of the Lower Link: This tends to shorten the effective beam length of the implement (which is the distance to the instant center T Fig. 14.4). The shorter beam length increases the lateral of the pull of lower link which provides the main component of the centering force on the tool.
 
2. Lengthen the Upper Link: This tends to decrease the decentering moment caused by the upper link about the instant center T. A longer link will decrease the rate at which the off set of upper link force from instant center T (i.e. V) is increasing as decentering occurs.
 
3. Shorten the lower links: This tends to increase the rate at which instant center T will move laterally as the hitch is decentered.
 
4. Shorten the lift link: As the hitch is decentered, the lift links become angled in the same direction. Hence, the lift link force in the lower link tends to recenter the hitch. Shorter are the lift links, more will be their tilt, thereby increasing the tendency to recenter the hitch.
 
5. Increase Leverage of Lift Links w.r.t. Lower Link Pivot Point: This means lift link should be located rearward on the lower links. This will increase the effect on the implement of the force created by the lift links force tending to pull the hitch back to center.
 
6. Decrease the Line of Pull of Free Floating Condition of the Hitch: This will increase the load in the lift links. The effect of the lateral component of the lift link force is increased thereby increasing center-ability.



1410.png
Fig. 14.4: Center-ability of the hitch system

7. Trailing Characteristics

The geometry of the hitch system in the horizontal plane influence the trailing characteristics as shown in Fig. 14.5. The trailing characteristics of a mounted implement should be such that a reasonably uniform width of cut is maintained when operating around a curve without adversely affecting the tractor steering. If the rear-mounted implement is not permitted any lateral movement w.r.t. tractor, the implement will cut to outside when operating on a curve and steering response may be poor because of the side forces introduced by the implement. These effects are particularly objectionable with a directional implement such as plow. A laterally swinging hitch gives easier steering, but the implement under-cuts on a corner A non-directional tool tends to move along the line of pull when the tractor is on a curve. The linkage adjusting itself so that BH is perpendicular to the radius drawn through H. The implement appears to be pulled from a point within small area A1. The directional tools, however, tend to go in the direction they are pointed, rather than in the direction of pull. In this case, the implement is pulled from virtual hitch point in the zone A2 and implement adjust itself so that A2H is perpendicular to the radius drawn from H. Since A2 is further forward than A1, directional tool cuts the corner even more than an implement free to move in the direction of pull. For ideal trailing of any implement around a curve, the horizontal hitch point should be on the tractor centerline, equal distant from the center of resistance of the implement (H) and center of pull of the tractor (D).

1411.png
Fig. 14.5: Trailing characteristics during operation around a curve

8. Hitch Lift Capacity and Stability

A good mounted hitch should be able to lift and transport the size of implement matching with tractor power is one of the design requirements. The upper limit of the lift capacity of a given tractor is related to the tractor’s front end stability moment and minimum reaction required at the front end for proper and effective steering. The lift capacity depends on design geometry of hitch system and hydraulic pressure available also. The lifting pressure requirements vary as the implement is lifted since the hitch geometry changes. It is desirable to have the lowest lifting pressure at the point the implement comes out of the ground where the greatest force is required. The instantaneous center (IC) must always be considered when evaluating lift pressure (Fig.14.6). The force in the lift link is calculated as given below:

     1412.png                                    ... (14.7)
           
Where:             LL       =Lift link force

                       MI        =mass of implement

                       E          =perpendicular distance of implement weight from IC

                       D         =perpendicular distance of LL force from IC

The IC will move forward as implement is lowered which changes implement moment and LL.

Transport stability is the controlling factor for the maximum implement moment that can be safely considered. The static implement and stability moments are (Fig. 14.6).

               1418.png                   ... (14.8)
              1419.png        ... (14.9)

Where:

IMS      = static implement moment

SMS     = static stability moment

 Rf       = static reaction on the front wheel of a tractor without implement

The dynamic implement and stability moments are:

                                  1415.png                                ... (14.10)
                                  1416.png                           ... (14.11)

Tractor implement system is stable so long as SMD IMD and the difference (SMD - IMD) will appear in the form of front wheel reaction. Then, from steering ability
 
   1420.png      …(14.12)                      
                                   


1421.png
Fig. 14.6: Hit
ch lift capacity and implement moment

Last modified: Monday, 7 April 2014, 6:19 AM