Two Spring Bogie Suspension Assembly for Multi-axle Vehicles With Variable Suspension Characteristics And Method For Changing Suspension Characteristics of Leaf Spring Assembly

Field of invention:

The present invention relates to a two spring bogie suspension assembly for multi-axle vehicles provided with adjustable means for desirably varying the effective spring mounting length and thereby changing its stiffness characteristic. The present invention also related to method for desirably changing the suspension characteristics of a two spring bogie suspension assembly for multi-axle vehicles.

Background of invention:

The chassis of an automobile is mounted on the axles through vehicle suspension systems. A basic object of any vehicle suspension system is to suspend the vehicle body above vehicle wheels and to isolate the vehicle body from road shocks, which may be in the form of bounce, pitch, roll or sway. Generally, leaf springs or air-springs are used active components in vehicle suspension systems. Air-spring suspension system involves a completely different technology, and therefore has not been discussed hereinafter.

The rear axle suspension system of multi-axle vehicles generally comprises of leaf spring assembly. Such leaf springs assembly featuring a stack of leaf springs having mounting couplings attached at opposite ends have been disclosed in US 2007/0057423.

Suspension systems for multi-axle vehicles with leaf springs can be categorized into two types:

a) Four-spring tandem axle suspension system which uses two springs on either sides of the rear axles , and

b) Two-spring bogie suspension system which uses a single spring on either side of the bogie.

Two-spring bogie suspension system provides higher load carrying capacity and better dynamic balance than four-spring tandem axle suspension system. The ability of two-spring bogie suspension system to respond to road articulations is also higher. Accordingly, automobile manufacturers prefer two-spring bogie suspension system over four-spring tandem axle suspension.

However, the disadvantage associated with two spring bogie suspension system is that it is not tunable, i.e., its spring stiffness characteristics cannot be altered desirably at will. In heavy commercial vehicles, such as trailers, suspension systems are tuned to bear the maximum load capacity of the vehicle. Therefore, when the trailer is travelling in unladen condition will have higher ride clearance than desired and as the stiffness of the suspension system is maintained at a higher value the vibrations and jerks arising from road undulations is transmitted to the vehicle frame and body. Therefore, to avoid these unwanted stresses on the vehicles and to provide better riding comfort, vehicles should be provided with suspension systems, particularly rear suspension systems, which are tunable.

Hence, there is a sincere need for a mechanism for desirably varying the stiffness characteristic of a two-spring bogie suspension system to reduce stress on the automobile frame and body arising out of vibrations and jerks generated by road undulations and patches.

Summary of Invention:

Spring rate or stiffness of a spring is the measure of the deflection experienced by a spring at a given load. Spring stiffness varies inversely as the cube of the length of the spring.

Stiffness equation of a spring is given by:
Stiffness = 32*E*I*SF/L3------------ (i)

Where E = material property
I = moment of Inertia of the spring
SF = stiffness factor
L= spring mounting length

For a leaf spring suspension for rear axles L represents the axle spread of the rear axle.

As shown in the above formula (i), the stiffness of a leaf spring suspension depends on moment of inertia, material properties and axle spread of the suspension system. Moment of inertia in turn is dependent on the thickness of each leaf, width of each leaf and number of leaves constituting the suspension system and is represented by the formula:
Moment of Inertia = {[(b-t)t3/12]+[3.14t4/64]} x N----------- (ii)
where,

b = width of each leaf spring
t = thickness of each leaf spring
N= number of leaf springs

In order to bring about desirable changes in stiffness of a two spring bogie suspension system at least one of the stiffness determining parameters i.e. E, SF, L, b, t or N, is to be varied, as easily construed from the foregoing two equations.

Once a two spring bogie suspension system is fabricated, parameters such as leaf geometry (leaf width and leaf thickness) and material property (E) become fixed and cannot be altered. It is possible to change the spring stiffness by altering the number of leaf springs; but such act would hinder stepping of leaves and hence not a feasible solution. The only way of changing the spring stiffness is by varying the axle spread. Constructionally a two-spring bogie suspension system impedes alteration of the axle spread of the rear axle since the distance between the axles is fixed.

Given the constraints discussed above, the present inventors recognized that in a two-spring bogie suspension system, the effective spring mounting length can be varied by changing the spring mounting position on axle seat. Accordingly, the inventors propose a novel two spring suspension assembly, for multi-axle vehicles, with adjustable means for desirably varying the effective spring mounting length and thereby changing its stiffness characteristic.

Accordingly, the present invention provides a two spring bogie suspension assembly for multi-axle vehicles with variable suspension characteristics comprising:

- a pair of leaf spring assemblies formed of a plurality of leaf springs held in stack relationship,

- a trunnion bracket on which each said leaf spring assembly is mounted by means of U bolts and a top clamp,

- a bogie bracket for engaging each said leaf spring assembly with the chassis frame of a vehicle,

- a pair of spring seat assemblies , supporting each said leaf spring assembly on either ends , each said spring seat assembly being mounted on one of the vehicle axles through an axle mounting bracket, wherein each spring seat assembly is provided with adjustable means for desirably varying the effective spring mounting length and thereby changing its stiffness characteristic.

According to a particular embodiment of the present invention said adjustable means for varying the effective spring mounting length is a two-part arrangement comprises of a moving member and a fixed member, said moving member translates along a guide-ways provided on said fixed member. The guide-way provided on the fixed member may be V-type, square type or dovetail type.

According to yet another embodiment of the present invention the translatory motion of the moving member along the guide-way is achieved by lead screw mechanism. The lead screw mechanism may be powered by a servo motor for automatic adjustment of effective spring mounting length.

According to a further embodiment of the present invention the translatory motion of the moving member along the guide-way is achieved by hydraulic cylinder mechanism.
Preferably, in the invented two spring bogie suspension assembly the leaf spring assembly is camel-back shaped.

The present invention also relates to method for desirably changing the suspension characteristics of a two spring bogie suspension assembly for multi-axle vehicles; said suspension assembly comprises of

- a pair of leaf spring assemblies formed of a plurality of leaf springs held in stack relationship,

- a trunnion bracket on which each said leaf spring assembly is mounted by means of U bolts and a top clamp,

- a bogie bracket for engaging each said leaf spring assembly with the chassis frame of a vehicle,

- a pair of spring seat assemblies , supporting each said leaf spring assembly on either ends , each said spring seat assembly being mounted on one of the vehicle axles through an axle mounting bracket and being provided with adjustable means for desirably varying the effective spring mounting length;

said method being characterized by sliding of said adjustable means towards or away from each other to vary the stiffness of corresponding leaf spring assembly desirably.

Although the present invention finds its applicability in automobile sectors, the mechanism of varying spring stiffness as proposed by the invention may be utilized in any systems where load is suspended over resilient mountings, such as machine beds.

Advantageously the invented suspension system as can be effectively used in electronically actuated dynamic stability control system wherein the vehicle load centre of gravity (e.g.) is monitored and corrected by changing the stiffness of LH of RH spring so the e.g. is maintained within the anti-roll zone.

Brief Description of the Drawings;

For better understanding an illustrative embodiment of the invention will now be described with reference to the accompanying drawings. It will however be appreciated that the embodiment exemplified in the drawings is merely illustrative and not limitative to the scope of the invention.

Figure 1 shows a schematic one side view of the invented two spring bogie suspension assembly for multi-axle vehicles.

Figure 2 shows a schematic view front view of invented spring seat assemblies with adjustable means.

Figure 3 shows a graphical representation of variation of spring stiffness with effective spring length.

Detailed Description of the Drawings:

Referring to figures 1 of the accompanying drawings the leaf spring assembly (1) on either side of the invented two spring bogie suspension assembly (2) comprises of a plurality of leaf springs (3) held in stack relationship by means of bolt-clips (4) . The leaf spring assembly (1) is mounted on trunnion bracket (5) by means of U bolts (6) and a top clamp (7). The leaf spring assembly engages with the chassis frame of a vehicle (not shown) by means of a bogie bracket (8). The leaf spring assembly (1) is provided with a pair of spring seat assemblies (9) on either end. Each spring seat assembly (9) is mounted on one of the vehicle axles through an axle mounting bracket (10).

Each is provided with adjustable means (11) for desirably varying the effective spring mounting length and thereby changing its stiffness characteristic, as shown in figures 1 and 2.

When the moving members (12), located on either ends of the leaf spring assembly (1), are moved towards each other the effective spring mounting length decreases and thereby the spring stiffness increases. Similarly, when the stiffness of the leaf spring assembly (1) is to be decreased the moving members (12) are moved away from each other.

Referring to figure 2 of the accompanying drawing, the adjustable means (11) is a two-part arrangement comprising of a moving member (12) and a fixed member (13), said moving member (11) translates along a guide-way (14) provided on said fixed member (13).

Either lead screw mechanism or hydraulic cylinder mechanism may be used to affect translation of the moving members (12) along the guide-ways (14).

The inventors conducted a number of tests to evaluate stiffness of invented suspension system when the effective spring mounting length is varied. The test results demonstrating the performance the invented suspension system at different effective spring mounting length is provided in Table I below.

Table-1

The graphical representation of the variance of the stiffness of the invented suspension with alteration of effective spring mounting length has been show in figure 3 of the accompanying drawings.

As already mentioned, the foregoing description is illustrative of the invention and not limitative to its scope, because it will be apparent to persons skilled in the art to device other alternatives embodiments without departing from the broad ambit of the disclosures made herein.

WI CLAIM:

1. A two spring bogie suspension assembly for multi-axle vehicles with variable suspension characteristics comprising:

- a pair of leaf spring assemblies formed of a plurality of leaf springs held in stack relationship,

- a trunnion bracket on which each said leaf spring assembly is mounted by means of U bolts and a top clamp,

- a bogie bracket for engaging each said leaf spring assembly with the chassis frame of a vehicle,

- a pair of spring seat assemblies , supporting each said leaf spring assembly on either ends , each said spring seat assembly being mounted on one of the vehicle axles through an axle mounting bracket, wherein each spring seat assembly is provided with adjustable means for desirably varying the effective spring mounting length and thereby changing its stiffness characteristic.

2. The two spring bogie suspension assembly for multi-axle vehicles as claimed in claim 1, wherein said adjustable means for varying the effective spring mounting length is a two-part arrangement comprises of a moving member and a fixed member, said moving member translates along a guide-way provided on said fixed member.

3. The two spring bogie suspension assembly for multi-axle vehicles as claimed in claim 2, wherein the guide-way provided on the fixed member of said adjustable means is dovetail type.

4. The two spring bogie suspension assembly for multi-axle vehicles as claimed in claim 2, wherein the guide-way provided on the fixed member of said adjustable means is V shaped.

5. The two spring bogie suspension assembly for multi-axle vehicles as claimed in claim 2, wherein the guide-way provided on the fixed member of said adjustable means is square shaped.

6. The two spring bogie suspension assembly for multi-axle vehicles as claimed in claims 2 to 5, wherein translatory motion of said moving member is achieved by lead screw mechanism.

7. The two spring bogie suspension assembly for multi-axle vehicles as claimed in claims 2 to 5, wherein translatory motion of said moving member is achieved by hydraulic cylinder mechanism.

8. The two spring bogie suspension assembly for multi-axle vehicles as claimed in claims 1 to 7, wherein said leaf spring assembly is camel-back shaped.

9. Method for desirably changing the suspension characteristics of a two spring bogie suspension assembly for multi-axle vehicles;

said suspension assembly comprises of

- a pair of leaf spring assemblies formed of a plurality of leaf springs held in stack relationship,

- a trunnion bracket on which each said leaf spring assembly is mounted by means of U bolts and a top clamp,

- a bogie bracket for engaging each said leaf spring assembly with the chassis frame of a vehicle,

- a pair of spring seat assemblies , supporting each said leaf spring assembly on either ends , each said spring seat assembly being mounted on one of the vehicle axles through an axle mounting bracket and being provided with adjustable means for desirably varying the effective spring mounting length, and said method being characterized by sliding of said adjustable means towards or away from each other to vary the stiffness of corresponding leaf spring assembly desirably.