Patent Application
20080304906
A system for clamping two parts is disclosed in which (1) the material of one of the parts is composed of a material which susceptible to creep deformation when subjected to thermal cycling and (2) galvanic corrosion in bolted joints of dissimilar materials is prevented or inhibited.
There is a desire to reduce the weight of engine and other powertrain components to improve fuel economy. To reduce weight, many engine structural components, which may have been made from cast iron and steel in the past, have been converted to aluminum, which weighs about one-third that of iron. Further weight reduction is possible by manufacturing these same components from magnesium, which is the lightest structural metal at two-thirds the weight of aluminum. There are two potential concerns with bolted cast magnesium component attachment points, namely magnesium's susceptibility to creep and mismatches of Coefficients of Thermal Expansion for the various materials present in a joint. Most magnesium alloys exhibit unacceptable levels of creep phenomenon under thermal load in automotive engine applications. This can result in reduction of clamping force or bolt load retention causing parts to lose alignment and/or functionality. Secondly, mismatches in the thermal expansion rates of the different materials that make up an attachment point may lead to loosening in the engagement attachment joint.
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Magnesium alloys, such as AE44, are available that can withstand higher temperature cycling due to the addition of rare earth elements compared to magnesium alloys like AZ91D. However, the alloys containing rare earth elements are more expensive, have reduced castability, and are more susceptible to corrosion. It is known, in the prior art, to overcome the loosening of the clamping force is such a system by using aluminum fasteners or expensive creep-resistant magnesium alloys containing rare earth elements. However aluminum bolts are more expensive than steel bolts, require different installation equipment in an assembly plant, are less robust than steel bolts, and are prone to thread damage during handling, stripping of the threads, cross-threading during assembly, and failure from over-torque situations. The inventor of the present invention recognizes that it is preferable to use magnesium alloys without such rare earth elements, if the problem of creep could be overcome. Similarly, it is desirable, in some applications, to use plastic and composite materials if the creep problem could be overcome.
To mitigate the effect of the creep, an insert is provided in the material that is susceptible to creep, such as magnesium. The material of the insert more closely matches the thermal expansion of the steel bolt and has a unique shape, which retains the magnesium casting, while allowing it to float or deform to the extent its volume changes during thermal cycling. The insert manages all of the clamp force while the unique taper geometry holds the magnesium casting in place.
A system for clamping a first member to a second member is disclosed in which an insert is coupled to the first member. The insert is made of a different material than the first member and has a cylindrical through hole with the centerline of the through hole roughly parallel to the axis of the insert. The exterior surface of the insert is roughly cylindrical along the length of the insert except near one or both ends of the insert where the outside diameter increases monotonically in the direction of the end of the insert. A bolt passes through the insert coupling with threads disposed in the second member. In one embodiment the first member is made of magnesium, the second member of aluminum and the bolt of steel. In alternate embodiments, the first member is made of reinforced plastic or a composite material.
The insert is cast into the first member. Alternatively, it is pressed into the first member. Or, in another embodiment, it is thermally sprayed into the first member. In the embodiment where the insert is pressed into the first member and the insert flares at both ends, the insert is made of two pieces to allow insertion into the first member.
The increase in diameter near one or both ends of the insert is linear with respect to length along the axis of said insert. Alternatively, the increase in diameter forms an arc tangent to the exterior surface to obviate stress risers in said insert.
In one embodiment, the insert sits proud of the first member, meaning that it protrudes above the first member so that the bolt head is in contact with only the insert and not the first member to prevent corrosion by galvanic action.
Also disclosed is a clamping system with a first member made from a material susceptible to creep under thermal loading. An insert is coupled to the first member. The insert is made of a different material than the first member and has a cylindrical through hole. The centerline of the through hole is roughly parallel to the axis of the insert. The exterior surface of is insert is roughly cylindrical along the length of the insert except near both ends where the outside diameter increases near the ends. A second member made from a different material than said first member is arranged to sit against both ends of said insert. A bolt is pushed into through holes in both sides of the second member and the insert. A nut threaded onto the bolt can be tightened to pull the whole system together. In one embodiment the second member wraps around the first member and is made from a single piece. In another embodiment, the second member is comprised of two pieces, one on each end of the insert. An isolating material can be placed between said first and second members.
An advantage of the present invention is that the magnesium casting is allowed to float under thermal loading, the thermally induced volumetric changes to the casting will cause less deformation in the neighborhood of the insert as that in a conventional joint of “straight” aluminum insert.
A further advantage is that the taper insert reduces the possibility of forming an electrical circuit. As a result, the aluminum tapered inserts mitigate potential effects of creep and galvanic corrosion.
Another advantage of the present invention is that current assembly tools and processes are not affected and there is no degradation of the robustness of the attachment points when switching from an aluminum bracket to a magnesium bracket. It also allows for the use of common, readily available magnesium alloys in higher temperature applications.
Yet another advantage is that in one embodiment, the insert is extended some distance beyond the surface of the magnesium casting to prevent galvanic corrosion between the steel bolt and the magnesium casting.
The advantages described herein will be more fully understood by reading an example of an embodiment in which the invention is used to advantage, referred to herein as the Detailed Description, with reference to the drawings wherein:
a-b show an attachment point according to prior art clamping systems, shown before (1a) and after creep (2a) due to thermal cycling;
a-d show non-limiting examples of the present invention utilizing an insert to mitigate the effect of creep on the attachment; and
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In an alternative embodiment shown in
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Insert 18 can be press fit into magnesium part 16 for the embodiment shown in
In the discussion above, the material of the elements are described as: upper part of magnesium with an insert of aluminum, bolt of steel, and the lower part of aluminum. However, these described materials are by way of example and not intended to be limiting. The present invention applies to any clamping system in which material creep compromises the clamping force.
The invention is not limited to any particular clamping system. However, the inventors envision using the clamping system to hold a magnesium bracket to an aluminum cylinder head. Additional embodiments include:
The list above is provided for purposes of illustration only and is in no way intended to be limiting.
A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.
