Four-Point Bearings Do Triple Duty While Saving Space
A four-point contact ball bearing makes it easy to simplify machine designs that combine radial, thrust and moment loads, because it can handle all three simultaneously. They are primarily used for slow-to-moderate-speed applications, or where oscillatory movement is predominant.
A single, four-point contact bearing usually makes a second
bearing unnecessary, which provides some very important
- Space savings
- Weight savings
- Cost savings
- Rigidity and accuracy
- Faster installation
- Less maintenance
- Integral mounting holes, gears, etc.
Thanks to a unique ball path geometry that produces twice as many contact points, a four-point contact bearing can resist radial, thrust and moment loads individually, or in any combination. It has one row of balls with an inner race and an outer race. The ball grooves are generated from two centers, with each radius slightly larger than the ball radius. Contact angles are typically 35° off the radial centerline, but can be varied to suit the application.
Four-point contact bearings are especially effective in applications in which the rotating element has a high ratio of diameter to axial length. Most have a large ratio of bore-tosection size: the larger this ratio, the thinner and more flexible are the individual races. Thin section bearing sizes range from 1"– 40"and slewing ring bearings range from 4" – 178" O.D.
Figures 1 and 2 show how much space can be saved by replacing two bearings with one four-point contact bearing. The rotary table design (Fig. 1) has a conventional center post supported by two bearings. Eliminating that center post (Fig. 2) reduces the height and weight of the table and frees space to be used by other components. And without a post, there is no chance of post deflection.
Similarly, having a single bearing means no runout matching, no diameter matching, and no chance of wobble from two bearings with unequal runout — all of which enhances the accuracy of the assembly.
Eliminating a second bearing also eliminates the need for adjustments to optimize its fit — and thus the expense of spacers or clamping rings (and perhaps technicians skilled in making these adjustments). In contrast, a four-point contact ball bearing is manufactured with the proper internal fit and is simply fastened to its mating parts.
The Four-Point Bearing: How it Works
As the inner race rotates, a downward thrust load is transferred to the ball at point B, and then transmitted through the ball (typically at a 35° contact angle) to point D. From there it passes into the outer race and support structure. Deflection of the ball in the race grooves along the load-transmission line relieves the ball at contact points A and C and allows the ball to rotate smoothly around an axis perpendicular to line BD.
Moment load is much the same, passing a thrust load from the inner race at point B, and to the outer race at point D. At the same time, a thrust load 180° opposite is passed from the inner race at point C to the outer race at point A.
Radial resistance occurs 180° from the point of application (assuming much larger thrust load than radial load) and the radial load is resisted equally across the lines of contact CA and BD. The relationship of the individual loads determines whether the reactions are large or small.
Finally, eliminating a second bearing saves money that can be used to integrate features (gears, mounting holes, lubrication fittings) into the four-point contact bearing to further optimize performance.
Twice as accurate. Four-point contact bearings can be manufactured in accordance with ABEC precision grades and in larger sizes. In applications involving oscillatory motion with limited angles of rotation — when the runout is zero for much of the rotation — an ABEC 1 bearing can sometimes be used instead of an ABEC 3 or 5. In such cases the critical requirement (axial runout or radial runout) should be specified to meet ABEC standards; noncritical areas can be left up to the bearing manufacturer.
With one row of balls instead of two, accuracy is enhanced in several ways. For example, when radial runouts in a tworow system are out of phase, the rotation will produce a wobble. Wobble can also result from structure deflection when applications require a lot of space between two rows of balls; e.g., when high-moment load must be resisted. And if there is a pre-load difference in axial runout between the two rows, bearing friction can vary.
None of these problems will occur in a bearing with a single row of balls. Likewise, there is no matching of housing bores and shoulders, or of shaft diameters and shoulders. All other factors being equal, the accuracy of a four-point contact bearing may be twice that of a bearing with two rows of balls.
Choice of Materials
Four-point contact ball bearings are typically manufactured of conventional anti-friction bearing materials but, like other bearings, materials decisions are ultimately dictated by the application. High-carbon 52100 alloy and 440C stainless steel are common choices for through- hardening. When selective hardening is needed, such as when a bearing has integral features, like flanges and gears, options include carburizing or induction hardening. Alloys such as 8620, 4340, 150, and 1552 are utilized for this purpose. All can be hardened to Rc 58 for 100% bearing capacity and maximum service life. (At Rc 50, capacity drops to about 50% and at Rc 45, to less than 30%.)
When corrosion resistance is important, races and balls can be made of precipitation-hardened stainless steels or coated with a thin — yet dense — chrome, such as Endurakote plating.
Four-point contact bearings are primarily used for slow-tomoderate- speed applications. Under combined loads, some of the balls are loaded across both sets of contacts. Since they cannot roll about two axes simultaneously, they tend to rotate about one axis. This leads to more skidding or slippage on the other set of contacts, increasing friction and wear — especially at high speeds.
Maximum allowable speeds vary with bore size, section size, ball size, retainer type, type of lubrication, and type of loading. As a rule, this type of bearing performs best at pitch line velocities under 1,100-feet-per-minute. However, the variable effects of preloading, lubrication and simultaneous individual loads can overcome this limitation. For applications requiring higher speeds, the bearing manufacturer should be consulted.
Under a single loading (radial or thrust), speed is no more a factor for a four-point contact ball bearing than that it would be for a conventional radial bearing or an angular contact thrust bearing.
Lubrication and Maintenance
In most applications, four-point contact ball bearings need no more maintenance than any other anti-friction bearing. Often, they need even less.
Grease lubrication is typical, since so many applications involve low speeds or oscillating motion. If the application involves full rotation, two grease fittings — mounted 180° apart — should allow sufficient access. But if a design calls for heavy loads as well as full rotation — or if the bearing is located near other moving parts that must be lubricated with oil — oil lubrication is used instead.
While many bearings are pre-lubricated at the factory, the lubricant must nevertheless be replenished and replaced as needed. The incorporation of holes or fittings into the bearing races facilitates this, which is especially true when the bearings have integral seals — a possibility even in large-diameter bearings (e.g., 100 inches).
Separators. Separators should always be used in fourpoint contact bearings, the style varying with the application. Material options include steel, stainless steel, bronze, phenolic, nylon, Teflon, and stainless steel wire (spring-type). The most common style is the retainer found in a Conrad deepgroove bearing, which can be inserted into the bearing from the side through the gap between the race lands.
A full, one-piece retainer is usually the strongest option, but inserting it into a four-point contact bearing requires splitting one of the races or providing a loading plug. This is not practical in light-section bearings because the race is too thin to split and cannot be reassembled with rivets or screws.
When a thin section bearing application demands maximum capacity in a slow-speed bearing and separation of the balls, a wire separator is a good choice. Spring-type separators are used in low-frequency, oscillating, or slow, full-rotational applications where low, uniform bearing torque is important. The springs act as free-floating bodies to keep sliding friction very low.
Today's design engineers are challenged to simplify designs in order to reduce cost and weight. In many applications, four-point contact ball bearings — which do the job of multiple bearings by handling radial, thrust and moment loads simultaneously — are an ideal solution.
For more information:
2860 McCracken Street
Muskegon, MI 49441
Fax: (231) 759-4102
Reprinted courtesy Kaydon Bearings.
The article "Four-Point Bearings Do Triple Duty While Saving Space" appeared in the March 2015 issue of Power Transmission Engineering.