Motion System Design
Silver Linings

Silver Linings

Slewing bearings offer an elegant solution to applications, large and small

Slewing rings are thin loops: large-diameter bearings with large bores designed to support axial, radial, and tipping force loads. Common applications include cranes, excavators, turnstiles, wind generators, telescopes, and tank turrets. However, slewing bearings are not just for heavy-section, low-precision applications anymore; improved manufacturing methods and design concepts are permitting their use in increasingly small, sophisticated, and precise applications. Both ball and crossed-roller configurations are common. The direction and degree of loading, especially tipping load, mounting arrangements, and performance requirements all determine which design is most appropriate. To combine the function of a bearing and gear, often teeth are cut into the inner or outer race to either rotate with the assembly or remain stationary with respect to ground during operation.

But let’s back up. Why are slewing rings so useful in the first place? The ability to quickly bolt rotating structures to stationary bases is a major design benefit. And in the machine tool industry especially, off-the-shelf availability of highprecision slewing bearings preloaded to eliminate clearance and vibration has become standard. Runout and diameter tolerance increments for these bearings range from thousandths to ten-thousandths of an inch (0.0001 in.) Compact slewing rings offer reduced cross sections and small diameters down to the 50-mm range. (These bearings are smaller than many of the balls used in traditional, heavy-section slewing rings.) This has enabled designers to rethink traditional bearing arrangements.

For example, king-post designs that utilize two (ball or roller) bearings adjusted against one another at assembly can in some cases be replaced by a single bearing. Traditional king-post bearing mounting arrangements used to support radial, thrust, and moment loads consist of two ball or roller bearings spaced along a common axis. Greater moment capacity is obtained by spacing the bearings farther apart, or by using heavier section bearings. However, the space and mounting complications with these arrangements do not lend themselves to all new applications. That’s where slewing-bearing designs come to the rescue. Slewingring rolling elements create a reactive moment within a single bearing’s envelope to oppose the applied overturning moment load. Replacing two bearings with one reduces the height required for an application — and can simplify assembly as well. As a result, slewing bearings are particularly well suited to newer manipulator designs in industrial automation.

Bearing selection

Newer design options include a variety of internal geometries as well. But one size doesn’t fit all here: There is no one best design. Selecting the right bearing depends on application load, stiffness, speed, size, and smoothness-of-rotation requirements. Joint reviews between the end user and manufacturer of design goals and bearing capabilities can help optimize system performance while minimizing cost and promoting trouble-free operation.

Without any past experience to go on, a good design to consider is a single four-point contact ball bearing. This bearing utilizes races shaped like gothic arches on both inner and outer race ball paths that generate four points of contact on each ball. Intersecting contact angles create a large effective pitch diameter to offset overturning moment loads. In addition, the use of a single larger-diameter bearing allows wiring and plumbing to pass through the bore of the bearing. This can simplify overall designs, improve their outward appearance, and protect components. Further, while four-point contact bearings have been used for many years in the construction equipment industry for cranes, backhoes, and excavators, these bearings are also used in a wide range of specialty equipment. This equipment includes robotics applications, lift-and-rotate tables, machine tools, aerial baskets, aerial platforms, and radar pedestals.

While load-versus-capacity considerations are usually determining factors in selecting bearing crosssection and diameter, other parameters can resolve which bearing type is best.

  • Larger torques may indicate the use of a two-row ball bearing in place of a four-point contact bearing.
  • Increased moment stiffness may require a roller bearing.
  • Maximum capacities may necessitate the use of either a cross-roller bearing (a two-row or three-row roller bearing) or an eight-point ball bearing.
  • Deflections in the mounting structure (or the use of aluminum, such as in the U. S.’s M1A2 Abrams Main Battle Tank) may dictate the need for an aluminum wire race instead of a steel ball or roller bearing. In fact, the use of aluminum bearing rings with hardened steel wires not only matches the coefficient of expansion of aluminum mounting structures, but can also provide needed flexibility to compensate for mounting distortions.

While larger-bore bearings may be necessary to increase stiffness and capacity, smaller-bore bearings may be necessary in weight-sensitive applications such as robotics and manipulators. Smaller-bore slewing bearings enable smaller mounting structures for overall weight reductions and reduced moments of inertia for faster movements with fewer structural deflections. Standard turntable bearings are commonly available in bore sizes ranging from under 145 mm to more than 800 mm. High capacity and especially compact size make them ideal in the joint positions of articulated systems. (And because most slewing bearings bolt on, they can also ease assembly and reduce the number of components in a system.) Reduced weight can lead to significant cost reductions due to smaller motor and brake requirements, lower operational costs due to reduced power consumption, and reduced maintenance expenses.

Weigh your options

  • Corrosion resistance. Looking sharp never hurts. Pin striping and fluorescent colors on excavators are not uncommon, while bondo and glossy paint can be applied to forklift counterweights for a bit of flash. However, special coatings can maintain bearing performance too. Thin dense chrome (TDC) or nickelspray coatings are often used for protection against corrosion, while special paints or black oxide for external surfaces can also provide long-term viability to equipment.
  • Lubrication and sealing. Lubrication and sealing considerations are critical. In fact, most bearing failures are related to improper lubrication or contamination problems. While the sight of grease is commonplace and accepted in many construction equipment applications (both with and without seals), grease leakage in other applications such as robotics, positioners, and manipulators is unacceptable. Would you buy a car if you looked under it and found leaking oil? Probably not. Leaking grease can ruin products and create safety issues in many applications.

For better or worse, some form of lubrication is almost always required in bearings to prevent wear and corrosion. Special greases and seals for wet environments, food processing, oscillating conditions, high speed or low torque applications may be required. In addition, solid lubricant and dry film options may simplify the maintenance and operational concerns for an application while extending its useful life.

  • Ball and roller separators. Rolling elements are often kept apart by individual spacers. This arrangement is widely accepted and usually successful in keeping motion smooth. Still, special situations sometimes require the use of special ball or roller separators. Bearings mounted with a horizontal axis or used for continuous rotation, marginal drive power, and tight positioning and repeatability may require strip-type ball separators to keep rolling elements in the proper circumferential position.
  • Gears. In many cases, an application’s drive system can be simplified by incorporating a gear on the bearing’s inner or outer race. These gears take on many styles and features: Fellows stub or full-depth involute, hardened or unhardened, and ground or unground. Bearing manufacturers and past experience are vital resources in obtaining an acceptable gear design. However, the use of an outside gear expert can also be helpful when specifying drive trains to avoid future problems. Considerations to verify design adequacy include gear static strength, resistance to pitting, gear fatigue, and gear/pinion interfaces.
  • Bolting options. Bolts are the primary means of attaching slewing bearings to the stationary and rotating members, so quantity, length, diameter, grade, and especially mounting is critical. Configurations vary not only from application to application but also from inner to outer race. Given the wide range of options and the importance of obtaining adequately bolted joints, careful and thorough consideration of all bolting arrangement aspects should be completed. Again, consultation with an expert is advised to verify joint adequacy and recommended tightening procedures.

Often the assembly procedure and accessibility dictate when a through hole is appropriate. Through holes allow bolts to be inserted through their slewing ring into threading on mating structures. One advantage to through holes is longer bolt-length to diameter (L/D) ratios, which improve tensioning characteristics of bolted joints.

C-bore holes allow bolts to be inserted though a slewing ring so that the bolt head is flush or sunk below the ring’s face. This is often required in tight mountings with small axial clearance between rotating and stationary members. However, special care should be taken when using C-bore holes to ensure that the individual components’ fillet radii and corner breaks do not interfere with one another, and that an adequate clamping surface exists between the bolt, washer, and Cbore mounting surface.

Blind taps allow bolts to be threaded into one side while not continuing through to the slewing ring’s smooth, opposite side. This is sometimes needed when a contact seal rides that opposite side, or when a smooth surface is desired for esthetic purposes. However, blind-tap holes can require a shortening of bolts. Because this can translate into smaller L/D ratios, these attachments should be carefully checked to make sure proper preloading (bolt torque) is obtained.

Through-tapped holes are generally used when it’s necessary to thread bolts into a slewing ring, but not detrimental to have holes break through to the opposite side. This allows quicker machining of holes (drilled prior to tapping threads) than possible with blind drilling. However, like blind holes, throughtapping can require shorter bolts with smaller L/D ratios that must be checked for proper bolt torque.

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