FIELD OF INVENTION
This invention relates to an improved outer tube used in suspension of automobiles. This is particularly related to outer tubes of the dampers whether they are Front Forks of two wheelers or aluminium outer tube of dampers / shock absorbers for any vehicle .
PRIOR ART
Outer Tubes made from Aluminium Casting or Extruded Aluminium are being used currently for Front Forks , Inverted ( Upside Down ) Front Forks and dampers with some special requirements
Currently the outer tube is either made of steel tube or from aluminum thru gravity die casting. The steel tubes are used only for economical front forks only where the aesthetics takes a back seat . Following are the disadvantages of using steel tubes in Front Forks as outer tubes :
1. Aesthetically poor .
2. Cumbersome and aesthetically very bad to have arrangement to mount calipers of disc brakes on steel tubes as some cleat etc has to be welded on to the steel tube for the same .
3. Being Steel they are prone to rust especially where the welding is done to generate profiles to mount calipers of disc brakes or fenders .
4. Outer tubes made of steel tubes are definitely higher in weight thus increasing the weight of the vehicle unnecessarily .
5. Since this has to function as outer tube it is supposed to be thick in section . High thickness tubes do not have a very smooth surface at its inner diameter as per the standard manufacturing process of the
tubes. Also this surface gets further distorted when the welding is done at its outer diameter to join various other parts to it . At the same time it is a costly and difficult process to regain a good surface finish and tight dimensional control on the internal diameter of the steel tube in the post weld processing .
6. The above limitation brings an inherent quality issue . Poor
finish on the Internal diameter of the outer tube will cause high static friction in the operation of front fork which is a major drawback for any Front Fork . Also, this will cause wearing of the mating parts very fast . Since the mating parts are securely integrated to the inner tube or the outer tube these cannot be replaced as spares easily in the market . As a result, the effective life of such Front Forks is very limited .
Alternately, aluminium / alumninum alloy outer tubes are used to overcome many of the above disadvantages. The current process of making such tubes is Gravity Die Casting for all standard Front Forks . In inverted or upside down forks where again aluminium / aluminium alloy outer tubes are being used the manufacturing process used is of extrusion for basic tube followed by machining of the whole profile . In many dampers where aluminium / aluminium alloy is used for outer tubes, the manufacturing process is either extruded aluminium tubes or thru Gravity Die Casting Process.
The use of aluminium / aluminium alloy as outer tube material poses following disadvantages :
1. Since Outer tubes of Front Fork is a load bearing member of
Front Suspension this is required to be structurally strong. It is for this
reason only that Gravity Die Casting is used as a manufacturing process in place of Pressure Die Casting to manufacture the outer tubes for standard Forks . The problem for GDC is that it is usually a manual process which has some inbuilt problems of variations. These variations, if not kept in control thru a strong system / discipline , can lead to breakages of outer tubes in field which would be detrimental & can lead to fatal accidents .
2. GDC is as such a process with slow rate of production which increases the cost of production .
3. GDC also does not allow many intricacies in the profile and hence the outer profile can be nothing but simple geometrical profiles .
4. A lot of machining allowance is kept in the casting owing to the variations in the GDC process which is not only a waste of material but also waste of machining time .
5. GDC process also leads to a less dense casting and also a
casting which has porosity in built . Porosity leads a high rejection rate
after machining which is definitely not required.
6. Less density causes another problem . When the Front Fork
operates on the vehicle, the inner tube ( made up of steel) slides against
the inner diameter of the outer tube . Low density of aluminum of the
outer tube leads to the erosion of its internal diameter which causes
following :
a. The eroded metal makes a sludge which hampers the
operation of front fork . This calls for frequent servicing of the Front Forks . In a vehicle where the rear shock absorbers are not serviced throughout their life the Front Forks are to be serviced approximately every 10,000 Kilometers of running for proper functioning .
b. Also erosion of internal diameter causes increase in the
internal diameter of the outer tube which leads shifting of inner tube to one side thus causing an unequal load on the oil seal . This leads to leakage of the oil seals frequently . Finally, a stage arrives when the Front Fork becomes inoperative and has to be replaced with a new one . 7. In case of inverted Front Forks the set of disadvantages are quite different and as follows :
a. These tubes are extruded tubes and hence they cannot be
profiled . Also since these tubes by default are long ones and the profile
variations are large it leads to a heavy expense of machining the same.
b. Also since lots of machining is to be done as mentioned
above , the input material weight is quite high as against the final weight
which makes this process quite inefficient.
c. Also as in case of inverted tubes, the outer tube has to
support heavy loads and much thicker section of the tube is used as a
design requirement . This increases the cost of the Inverted Front Fork
drastically.
d. Since aluminum cannot sustain the heavy stress of steel
forged under bracket the under bracket of Inverted front fork is made
forged aluminum again increasing the cost of inverted front fork
e. Again as it is to machine the outer tube the intricate
shape on the outer tube can't be generated.
OBJECTS OF THE INVENTION
The primary object of the present invention is to provide an improved outer tube used in suspension of automobiles which overcomes disadvantages associated with the prior art(s}.
Another object of the present invention is to provide an improved outer tube used in suspension of automobiles which is simple in construction.
Further object of the present invention is to provide an improved outer tube used in suspension of automobiles which is efficient.
SUMMARY OF INVENTION
This invention is directed to the outer tube used in suspension parts such as Front Forks or Dampers . Wherever the outer profile has some intricate shape aluminium / aluminium alloy casting is used for making such outer tubes . However, aluminium / aluminium alloy casting has some in built disadvantages and hence a composite outer tube with aluminium alloy casting on the outer profile and a steel / metal tube encapsulated in the core is being proposed here .
Aluminium / Aluminium Alloy tube with steel tube or any other metal tube encapsulated for Outer Tube of Front Forks or any Damper is essentially an aluminium casting on the outer profile and steel or any other metal tube on the inner side. All the profile features are generated thru aluminium casting which can be processed either by Pressure Die Casting or a Gravity Die Casting process . The encapsulated tube can be of steel or any other metal which can be chosen to have the advantages of that metal over aluminium for the functions desired at the internal diameter of Outer Tube and also to provide strength and other physical properties to the outer tube which are better than the aluminium casting tube of the present art.
This present invention is directed to a design with steel or any other metal tube encapsulated in the aluminium / aluminium alloy outer tube . This design has advantages of both steel or the metal tube and aluminium / aluminnium alloy .In this case the steel tube / other metal will be casted along with aluminium / aluminium alloy tube' . The casting process can be Gravity Die Casting or Pressure Die Casting . The economics and technical preference will go for Pressure Die Casting. The composite outer tube, aluminium casting with steel or any other metal tube encapsulated, can also have sleek profile with several profiling options on the outer surface. This is also very cost effective especially in case of inverted front forks.
The advantages of the encapsulated steel / other metal tube in aluminium outer tube are explained herein below :
1. The steel / other metal tube is encapsulated inside the
aluminium outer tube . It implies that, the inner diameter of the
composite outer tube will be of steel / other metal while the outside
profile will be of aluminium/ aluminium alloy . Steel or any other metal
chosen as encapsulated tube will have high wear resistance . This will
give an advantage of comparatively very less wear of the material from
internal diameter of the outer tube .
2. The above will result into almost a " service free " Front Forks . There will be no need of servicing of Front Forks for a very long duration as no sludge will be formed which would hamper the working of Front Fork .
3. Since a thin steel / other metal tube with a very nice inner surface finish will be casted along with aluminium/ aluminum alloy tube
there would be no need of any machining as in -case of aluminum/ aluminium alloy tube . This drastically reduces the cost of the outer tube as well time of conversion and variations of machining . Also as the steel tube is not subjected to outside welding there would be no distortion of the encapsulated steel tube .
4. The casting process in this case can be chosen as Pressure Die
Casting as all the load requirements will be taken care by the
encapsulated steel tube . Pressure Die Casting will allow the following
natural advantages to this composite outer tube
a. the productivity will be very high as compared to Gravity
Die Casting process employed these days to make aluminium outer tubes
b. it will be possible to have intricate profiling of the outer
tube
c. it will be possible to vary little machining allowance for all
the remaining portions requiring machining of Outer Tube (the internal
diameter needs no machining as discussed earlier ) .
d. It is a mechanized process where variations in the process
can be effectively controlled . Also, any small non conformities generated
in the process will not have any detrimental effect on the performance of
Front Fork and hence the vehicle.
5. Since there is no major wear of internal diameter of the
composite outer tube the internal diameter will not change much with
usage . This will in a way cause inner tube to run coaxially and thus oil
seal wear will be uniform leading to a long life of oil seal..
STATEMENT OF INVENTION
According to this invention, there is provided an improved outer tube used in suspension of automobiles comprising of an outer tube provided with an encapsulated inner tube wherein said outer tube is made of Aluminium/Aluminium alloy and inner tube is made of a metal such as steel.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Further objects and advantages of this invention will be more apparent from the ensuing description when read in conjunction with the accompanying drawings and wherein:
Fig 1 shows the Front view of Leg Assembly of Front Fork having the encapsulated steel / other metal tube in aluminium / aluminium alloy Outer tube and an isometric view of the same leg assembly in detail.
Fig 2. Shows the cut section of the Leg Assembly of Front Fork shown in Fig 1.
DETAIL DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS:
Referring to fig. 1 wherein the present invention has the encapsulated steel / other metal tube ( part no 2 ) in aluminium / aluminium alloy Outer tube ( part no 1 ) . There are two soft bushes namely guide bush (
part no 6 ) and slide bush ( part no 4 ). There are other standard parts of leg assembly of Front Fork shown for completeness such as main spring ( part no 5) , cylinder ( part no 3 ) , oil seal ( part no 7 ) , dust seal ( part no 8 ) , Plug ( part no 10 ) and O Ring in plug ( part no. 9 ).
Reference may be made to Fig 2 which shows clearly how the steel tube / other metal tube is encapsulated inside the aluminium / aluminium alloy outer tube. The inner tube sub assembly (part no. 9 ) slides against the internal diameter of Outer Tube ( part no. 1 ) . Since there is wear resistant encapsulated steel / other metal tube ( part no. 2 ) on the internal diameter , the inner tube slides against the same. A guide bush ( part no. 6 ) is mounted on the ID of the outer tube and a slide bush ( part no. 4 ) is mounted on the inner tube sub assembly . These bushes are made up of softer material . The inner tube slides on the wear resistant internal diameter of the outer tube thru the soft slide bush at bottom and thru soft guide bush at the top . These bushes which are quite cheap as compared to complete Front Fork can be replaced easily after they wear. The outer tube will now have a very long life and would not need any replacement throughout the life of vehicle. Also this will never break in the field under normal use . In case of accident also, it would at the most bend but will still keep the vehicle and wheel linked together
It is to be noted that the present invention is susceptible to modifications, adaptations and changes by those skilled in the art. Such variant embodiments employing the concepts and features of this invention are intended to be within the scope of the present invention, which is further set forth under the following claims:-

WE CLAIM:
1. An improved outer tube used in suspension of automobiles comprising of an outer tube provided with an encapsulated inner tube wherein said outer tube is made of Aluminium/Aluminium alloy and inner tube is made of a metal such as steel.
2. An improved outer tube used in suspension of automobiles as claimed in claim 1, wherein a guide bush is mounted on the ID of the outer tube and a slide bush is mounted on the inner tube such assembly.
3. An improved outer tube used in suspension of automobiles as claimed in any of the preceding claims, can be manufactured by gravity die casting or pressure die casting preferably pressure die casting.
4. An improved outer tube used in suspension of automobiles substantially as herein described with reference to the accompanying drawings.