Metal deforming – Process – Tube making or reshaping
Reexamination Certificate
1999-09-13
2001-06-19
Larson, Lowell A. (Department: 3725)
Metal deforming
Process
Tube making or reshaping
C464S183000
Reexamination Certificate
active
06247346
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to methods of forming automotive truck and other vehicular drive shafts which transmit torque from one point, such as a transmission, to another, such as a front or rear axle assembly. More particularly, the invention relates to methods of forming aluminum tubular members used in making such drive shafts and to a method for producing such.
BACKGROUND OF THE INVENTION
Drive shafts are commonly used in rear drive and four-wheel drive automobile and truck power trains and other vehicle drive trains to conduct torque and power from an engine or transmission located at one point in the vehicle to another site such as an axle assembly which can be rearward. Other applications include boating, farm or industrial power take-off shafts. Steel tubes have been used over the years and have generally proved reliable and effective. However, aluminum drive shafts offer advantages in weight and inertia savings if certain disadvantages in cost can be overcome.
One of the factors influencing the design of such a drive shaft is critical rotation speed, which is a function of the natural resonance of the shaft. If the critical speed of the drive shaft is within the rotary speed ranges encountered in normal use of the vehicle, especially under steady state conditions (e.g., cruising), such can result in noise vibration harshness (NVH) conditions. If the encounter with critical speed is transitory or brief, it may be tolerable whereas if it occurs at cruising speed, it is less tolerable. Increasing the drive shaft stiffness increases the resonance frequency and critical speed to such a high level that it is not encountered in normal cruising speeds which alleviates the NVH problem. An increase in diameter of the tube can stiffen the tube adequately but, in turn, can add weight unless the wall thickness of the tubing is made correspondingly thinner; but this, in turn, can require a higher strength material or a more costly composite.
One aluminum alloy that has been used for drive shaft applications is alloy 6061 which, according to the Aluminum Association (AA) registered limits, contains 0.8 to 1.2% Mg, 0.4 to 0.8% Si, 0.15 to 0.4% Cu, 0.04 to 0.35% Cr, the balance being aluminum and incidental elements and impurities. Some drive shafts made of 6061 are speed restrictable, and their use can involve a governor to reduce the drive shaft speed and avoid the drive shaft's critical speed. The use of longitudinal graphite fibers pultruded on the outside of a 6061 alloy drive shaft tube provides a composite drive shaft that is stiffer, such that it can spin or rotate faster, but this obviously adds to the cost of the drive shaft.
Another important aspect of a drive shaft is that it effectively transmits quite substantial amounts of torque. For instance, in a lightweight truck or a large automobile, the engine torque might be 350 pound feet at the engine crank shaft, but that can be elevated to a much higher level through a multiplying transmission (in a low gear) to a torque level of 1000 or even somewhat higher, for instance 1400 or 1500 pound feet of torque at the transmission output. Transmitting this torque obviously places a strength demand on the drive shaft in addition to the critical speed-vibration limitations. Aluminum drive shafts have been reported to make noises when the vehicle is placed into gear.
Typical drive shafts can range from about 2 or 3 inches in outer diameter (O.D.) to about 4½ or 4¾ inches or even higher, especially for trucks, for instance up to about 5 inches O.D. or even more, such as from 5½ to 7½ inches or more. Typical wall thicknesses are within about 0.05 or 0.06 inch up to about 0.08 or 0.09 or 0.1 inch or even thicker, for instance up to about 0.13 or 0.14 inch or 0.16 or 0.17 inch or more, for instance 0.25 inch. A typical drive shaft for an automobile could have an O.D. of about 3.5 inches and a wall thickness of about 0.08 inch, whereas a drive shaft for a truck could have an O.D. of about 4½ or 5 inches and a wall thickness of around 0.07 to 0.09 inch and a typical medium duty truck drive shaft can have an O.D. of 7 inches and a wall thickness of about ¼ inch.
In addition to the performance demands on the drive shaft, the material selected for a drive shaft needs to be readily capable of the fabrication steps employed in making a drive shaft, which can include welding yokes at each end for universal joints which, in turn, requires that the material selected be weldable and that it have good strength capability after welding.
An object of the present invention is to provide an aluminum alloy drive shaft that is quiet during periods of high torque.
SUMMARY OF THE INVENTION
In accordance with a preferred embodiment of the invention, disclosed is a method of forming a drive shaft having at least two cylindrical sections having different diameters and a transition section between the two cylindrical sections. The method includes (a) providing an 6000 series type alloy hollow elongate tube; and (b) reducing the diameter of at least one portion of the hollow elongate tube to form a reduced diameter section and transition section between the reduced diameter section and the tube; the transition section having at least three subsections:
i. a first subsection having a first slope;
ii. a second subsection having a second slope; and
iii. a third subsection located between the first and second subsections having a third slope which is less than the first and second slopes, the third section forming a circumferential step to stiffen the transition section.
The drive shafts can also be made of metals other than aluminum alloys.
In accordance with a preferred embodiment of the invention, an aluminum alloy containing about 0.5 to 1.3% Mg, about 0.4 to 1.2% Si, about 0.6 to 1.2% Cu, about 0.1 to 1% Mn, the balance substantially aluminum and incidental elements and impurities, is employed to make improved vehicle power transmission drive shafts at reduced cost over some other approaches, especially when considering the drive shaft designs that require high operating capability in terms of relatively high rotation speeds and relatively high torque transmission. The use of the tubing made in accordance with the invention enables producing vehicular drive shafts having the desired characteristics.
The aforesaid preferred alloy for the invention drive shafts includes AA alloy 6013 which is described in U.S. Pat. No. 4,589,932, the entire content of which is fully incorporated herein by reference. The Aluminum Association composition limits for alloy 6013 are 0.6 to 1% Si, 0.8 to 1.2% Mg, 0.6 to 1.1% Cu, 0.2 to 0.8% Mn, 0.5% max. Fe, 0.1% max. Cr, 0.25% max. Zn, 0.1% max. Ti, other elements 0.05% each, 0.15% total, the balance substantially aluminum. The entire content of the Aluminum Association “Aluminum Standards and Data”, 1993, is incorporated herein by reference. All composition limits herein are by weight.
In accordance with the invention, the improved drive shaft stock is preferably made by an improved process including extruding and drawing to better facilitate consistent good strength properties including ultimate (breaking) tensile strength and yield strength, along with quite good elongation and quite good workability. This is attributed, at least in part, to achieving in the drawn tube a relatively uniform or consistent recrystallized grain structure as described herein. If care is not exercised in producing the desired extruded and cold finished tube, large or relatively widely varying grain sizes can occur when the metal recrystallizes during heating such as during an anneal or a solution heat treatment. Such grains of large or relatively widely varying size can detract from strength properties by either lowering them or making them inconsistent from extrusion to extrusion or even along the length of a given extrusion, or combinations of these effects. It can be desired to have the tube unrecrystallized such that most, for instance 70% or more, or substantially all of the tube met
Alcoa Inc.
Larson Lowell A.
Pearce-Smith David W.
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