FEATURE BEARINGS Simplifying machine design
With the ability to handle radial, thrust and moment loads simultaneously, four-point contact bearings can help to simplify the design of machines. Rick Burgess and Dave VanLangevelde of Kaydon Bearings comment
I
n industry today, design engineers are often challenged to simplify machine
designs in order to reduce weight and cost. When it comes to bearings, the use of four-point contact ball versions can help as they offer a combination of radial, thrust and moment loads simultaneously. Four-point contact ball bearings are
primarily used for slow- to moderate- speed applications, or where oscillatory movement is predominant. And, a single four-point contact bearing usually makes a second bearing unnecessary. Featuring a 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 centres, with each radius slightly larger than the ball radius. Contact angles are typically 35˚ off the radial centreline, but can be varied to suit the application. These bearings are beneficial in
applications in which the rotating element has a high ratio of diameter to axial length. Most have a large ratio of bore to section size: the larger this ratio, the thinner and more flexible the individual races. Thin section bearing sizes range from 1” to 40” and slewing ring bearings range from 4” to 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 in Figure 1 has a conventional centre post supported by two bearings. Eliminating that centre post (Figure 2) reduces the height and weight of the table and frees up space. Similarly, having a single bearing
means no runout matching, no diameter matching, and no chance of wobble from two bearings with unequal runout, enhancing the accuracy of the assembly. Eliminating a second bearing not only saves cost but it eliminates adjustments to optimize its fit, and thus the expense of spacers or clamping rings. In contrast, a four-point contact ball bearing is manufactured with the proper internal fit and is simply fastened to its mating parts. In addition, eliminating a second bearing saves money that can be used to integrate
30 MAY 2015 | DESIGN SOLUTIONS
features into the bearing to further optimize performance. 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. Non-critical areas can be left up to the bearing manufacturer. The use of one row of balls
THE APPLICATIONS Four-point contact bearings are primarily used for slow to moderate-speed applications. As they cannot roll about two axes simultaneously, they tend to rotate about one axis, which leads to skidding or slippage on the other set of contacts, increasing friction and wear. Maximum allowable speeds vary with
Figure 1
bore size, section size, ball size, retainer type, and type of lubrication and loading. This type of bearing performs best at pitch line velocities under 1,100ft/min. The variable effects of preloading, lubrication and simultaneous individual loads can overcome this limitation. 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. In most applications,
Figure 2
instead of two also enhances accuracy. For example, when radial runouts in a two-row system are out of phase, the rotation will produce a wobble. This can also result from structure deflection when applications require a lot of space between two rows of balls. If there is a preload 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. In addition, the accuracy of a four-point contact bearing may be twice that of a bearing with two rows of balls. Although the bearing material ultimately depends on the application, four-point contact ball bearings are typically manufactured ins conventional anti- friction bearing materials. 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, and alloys such as 8620, 4340, 4150 and 1552 are used here. All can be hardened to Rc 58 for 100% bearing capacity and maximum service life. When corrosion resistance is
important, races and balls can be made of precipitation-hardened stainless steels or coated with a thin dense chrome, such as Endurakote plating.
Space can be saved by replacing two bearings with one four-point contact bearing
four-point contact ball bearings often need less maintenance than other antifriction bearings, and grease lubrication is typical. 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. Furthermore, separators should always be used, with material options including steel, stainless steel, bronze, phenolic, nylon, Teflon, and stainless steel wire (spring-type). The most common style is the retainer found in a Conrad deep-groove 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. In light-section bearings, however, the race is too thin to split. 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. R.A. Rodriguez is the trusted supplier of Kaydon bearings in the UK.
R.A. Rodriguez
www.rarodriguez.co.uk
Enter 228 / DESIGNSOLUTIONS
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