Design

Structural Bearings

COSMEC XL™ structural bearings provide a positive means to permit expansion and contraction of a superstructure relative to its substructure when subjected to temperature changes or seismic movement. The low coefficient of friction of COSMEC XL™ bearings minimizes the effects of thermal and vibrational stresses that might build up in a structure.

COSMEC XL™ structural bearings lend to simplicity and economy of design, manufacture, and installation. A typical assembly consists basically of only three components: the sole plate, the base plate, and the COSMEC XL™ self-lubricating plate, which separates them and permits relative motion between the two.

Designed to outlast the structure they support, the overall low cost and permanence of metal are factors that provide you with the advantages of bearings that have stood the test of time.

COSMEC XL™ structural bearings are normally provided with the advantageous “trepanned” style of lubricating pattern. Compared with the ordinary recesses and based on the same percentage of lubrication on a surface, the trepanned pattern permits the lubricant reservoirs to be closer together, as well as providing overlapping lubrication coverage both transversely and longitudinally (see details below).

With proper design and selection of accessory materials, COSMEC XL™ bearings will outlast the life of the structure they support. Rust and corrosion of mating materials have often limited the life of a bearing assembly to the life of the steel components, not the self-lubricating bearing.

A stainless steel mating surface on the sole and/or the masonry plates in contact with the self-lubricating surfaces is recommended in order to provide an entirely corrosion-resistant bearing assembly.

Expansion assemblies should be designed so that the loading is uniformly distributed over the entire surface of the bearing. If eccentric forces might tend to develop an edge concentration of loading conditions, some self-aligning features should be introduced for best results.

The mating plates with the COSMEC XL™ self-lubricating surface should be wider or longer than the COSMEC XL™ plate by at least the expected amount of motion. This protects the lubricated surface from exposure to possible contaminants when movement takes place.

COSMEC XL™ plate thickness should be consistent with the overall plate size. However, on flat plates, a minimum thickness of ½ inch (13mm) is desirable to provide a suitable thickness for machining and lubrication.

For Rotex™, Bi-Radial™, and Unisphere™ expansion plates, where both the flat and the radius faces are lubricated, a minimum thickness of 1 inch (25mm) is desirable. Also, a differential between maximum and minimum thickness of at least ½ inch (13mm) is strongly recommended. This differential provides the minimum necessary positioning power of the radial face in its mating member and translates the horizontal forces to the flat face for expansion and contraction movements.

The selection of radii is developed empirically. However, the selection is governed by the requirement of “minimum differential”. A ½ inch (13mm) minimum should be generated. On Rotex™ and Bi-Radial™ expansion plates, for maximum economy, the radius should be between 12” (305mm) and 36” (915mm) for the convex style and between 12” (305mm) and 24” (610mm) for the concave style. The 12” (305mm) and 18” (457mm) radii will cover the majority of requirements.

COSMEC XL™ expansion plates are supplied completely finished and ready for installation. Therefore, if regular or countersunk bolt holes are required, this information must be provided.

This will permit the proper layout of the lubricating pattern around the bolt holes for maximum efficiency. Bolt holes and the area consumed by countersunk recesses should be kept to the minimum practical for positioning and minimum loss of lubricating area.

Other methods of affixing the expansion plate, when necessary, are with the use of milled channels in the mating plate, building a “nest” by means of welding bars to the mating plate or by drilling and/or tapping partially-through holes into the “plain” or back face of the COSMEC XL™ expansion plate.

Self-Lubricating Bushings

COSMEC XL™ self-lubricating bushings were developed to fill the void in the field of bearings for service at slow speeds and heavy loads with permanent self-lubrication. The ability to withstand extremes in static loads, environmentally adverse conditions, high temperatures, and negligence are added “pluses” built into COSMEC XL™ bushings.

Design techniques for COSMEC XL™ bushings follow the basic principles for configuration that have been long established, with certain modifications. Consideration must be given to unit loading, surface speed, ambient temperature, and environmental media.

The design process consists of determining the bushing size, the bushing alloy, appropriate clearances and fits, the support system, and the proper selection of the lubricant. Load-carrying capacity is inversely proportional to the surface velocity (speed) and the operating temperature.

Please contact Cosmec’s engineering staff for any further help in the design of COSMEC XL™ bushings.

Design Coefficient of Friction

Friction is defined as the resistance to relative motion between materials in contact and is directly proportional to the shear strength of the softer material. The major causes of this resistance to movement are the interlocking action of the surface asperities and the high shear strength of the mating materials.

Metals, regardless of the degree of surface finish, contain surface irregularities or asperities that can be measured in micro-inches. In an unlubricated system of dissimilar materials, the softer material must necessarily wear or abrade if motion is to take place.

The purpose of a lubricant is to eliminate or minimize the actual contact between opposing materials and, at the same time, introduce a low shear strength lubricating material into the system.

COSMEC XL™ lubricants allow us to offer frictional coefficients of under 10%. However, for design purposes, a design coefficient of friction of 10% is recommended. This value has proven satisfactory for the majority of applications. Actual full-scale tests in the laboratory and in the field have determined that the COSMEC XL™ bearings have a coefficient of friction of between 4% and 8% for medium and heavy load applications.

For high temperature service and for lightly loaded applications, a design coefficient of 15% is recommended.

In critical applications where a low value for the design coefficient of friction is essential, COSMEC XL™ lubricants are available to meet this need. Please submit application details for recommendations.

The proper surface finish of the contiguous moving members is an important factor in the design of a bearing assembly. COSMEC XL™ bearings do not require expensive or highly polished finishes on the bearing surface or on the mating surface. An ordinary finish of 63 – 125 micro-inches RMS will provide satisfactory results with all COSMEC XL™ bearings.

Irregularities of surface textures are a divergence from the geometric ideal and can be described as peaks and valleys. With thin film lubricants, these peaks and valleys (asperities) must, of necessity, be small, and hence the finish must be high in order not to break through the film, causing metal-to-metal contact and high frictional forces.

Thick film COSMEC XL™ lubricants fill the asperities of the mating surfaces with high load-carrying capacity but low shear strength lubrication, forming a mirror-like friction-reducing film.