Motion System Design

Ball bushing bearing withstands misalignment

The latest child in a ball-bushing family of fruitful ancestry is being introduced to European markets right now. This baby and its shaft can be out of line with respect to each other without adding stress. The reason, say the proud parents: It is smart. And it will be here soon

The Super Smart Ball Bushing bearing by Thomson Industries Inc. steps beyond the manufacturer’s Super Ball Bushing concept; it recognizes types of misalignment and self-aligns around any or all of three axes independently to account for them. The new linear bearing is being offered overseas now in metric dimensions and ratings. You can be sure it will hit American markets soon. The manufacturer isn’t saying exactly when, but the smart money says it will be this year. And a ready market it may be, because the new bearing provides load capacities that rival the manufacturer’s ProfileRail systems without giving up the ability to absorb torsional misalignment with no increase in stress levels.

The new linear bearings, Figure 1, offer 6 times the load capacity or, at constant load, 216 times the life compared with conventional Ball Bushing bearings. And it offers double the capacity or eight times the life of the maker’s Super Ball Bushing bearing. (As with radial ball bearing life, linear bearing life varies with the cube of capacity for a given load.)

Do it itself

Each double-track bearing plate individually self-aligns on three axes, compensating for three types of misalignment:
• It compensates for shaft angular deflection or misaligned housing bore (that is, for “pitch,” Figure 2).
• It evenly distributes load on its two ball tracks (compensating for “roll,” Figure 3).
• It rotates on a radial axis to eliminate skewing between ball tracks and shaft (“yaw,” Figure 4).

Self-alignment minimizes friction, which helps performance and bearing life and simplifies installation.

The ring provides a hardened, axially curved surface to back the bearing plates and distribute the load imparted to the housing over the largest area possible. This helps maintain bearing- to-shaft diametrical fit-up when installed in a soft metal or nonmetallic housing.

The new bearing also introduces a double- lip floating integral wiper, which keeps out contaminants and retains lubricant without substantially increasing bearing friction.

Ease of use

Initially, both open and closed-type bearings are available in working bores of 16, 20, 25, 30, and 40 mm (1 mm 50.0394 in., approximately). Single-bearing maximum dynamic loads are 2,200, 4,000, 6,700, 8,300, and 13,700 N, respectively. (The newton, “N,” is a metric force unit. 1 N 5 0.225 lb, approximately). An open bearing is for use with a rail supported over its full length. It has an axially oriented discontinuity (or “cutout”) to allow rail support. Open bearings in this size range have eight tracks of recirculating balls; closed bearings, ten tracks. The double-track bearing plate design reduces the number of plates compared to single-track bearing plate designs. The latter may require that five plates be loaded at once, which means the housing bore must be extremely round to ensure that each of the five plates contacts the housing and supports its share of the load. The double-track bearing-plate design of the Super Smart Ball Bushing bearing requires that only two plates bear the load at once. Therefore, minor variations in housing bore don’t affect the bearing’s ability to carry maximum load. Thus, the roundness requirement on housing bores for double-track plates is less than that of single-track plates for bearings of similar load capacity.

The double-track design produces high capacity and long life in a compact arrangement. It provides the optimal number of balls per recirculating track and optimal positioning of tracks around the round rails because of the universal selfalignment of plates.

The new bearing accepts slight misalignment from any sources:

• Imperfections in housing-bore roundness and parallelism.
• Deviation in flatness of mounting surfaces.
• Imperfect system assembly.
• Deflection at load.

Bearing ratings are based on use with the manufacturer’s LinearRace shafting, which has a hardness of 60 to 65 Rc. For softer shafts, capacities must be derated. Load and life calculations are similar to those of most ball-type rolling-element linear bearings:

WR = P/(KΘ)(KS)(KL)


Lm = (KuKSW/P)3 x 105


W = Dynamic load rating from manufacturer’s catalog

WR = Required dynamic load capacity, N
P = Resultant of externally applied loads, N
= Factor for direction of resultant load (highest, 1.0, when that load is directly between two plates of a closed bearing and lowest, 0.73, when resultant load is directly perpendicular to a bearing plate)
= Travel life, m
= Shaft hardness factor (less than 1.0 for hardness less than 60 Rc)
= Travel-life factor (less than 1.0 for desired total travel exceeding 100,000 m)

The manufacturer supplies charts of values of all three factors.

The KΘ value is lowest for the situation of an open bearing with the load in pulloff configuration; that is, with the load directly opposite the opening, attempting to pull the opening laterally across the shaft.

Due to the Super Smart bearing’s higher load capacity, it can serve in applications where ProfileRail systems had been the only choice. The higher capacities let design engineers use the bearings in more confined spaces and at higher loads than previously possible.

Typical applications include robotics; packaging and food processing equipment; printing and marking machines; plotters; electronic assembly including surface mount technology; health-care, exercise, rehabilitation, and recreational equipment; inspection equipment; machine tools and fixtures; industrial lasers; and positioning systems of all kinds.

The manufacturer anticipates a lot of retrofit activity because of the significant gains in capacity and life, and is preparing for distributor sales activity accordingly.

Ball Bushing flashbacks

A device to enhance maneuverability of the propeller-driven aircraft of the pre- 1950s kept the propeller at constant speed even in a dive or climb, where forces resulting from the acceleration of gravity try to change propeller speed. Until the 1940s, constant-speed propellers were costly, electrically actuated items used mostly on larger aircraft. John B. Thomson, a flight enthusiast at age 11 and licensed pilot at 16, acquired an interest in a mechanically actuated constant- speed propeller in the early 1940s. However, it called for a sliding bushing and the bushing experienced stick-slip, making performance unreliable. The need for a rolling-element linear bearing was keenly felt.

The Ball Bushing bearing was conceived in 1944 and the first patent application was filed in January 1945 by Mr. Thomson and Hulbert K. Ferger. Mr. Ferger knew rotary bearings through work on multispindle drill heads. He suggested the idea of recirculating ball bearings for linear motion. As soon as the war ended, Robert C. Magee left Wright Field to join Thomson and Ferger and work full time on the bearing.

With time, the three found that the potential application range for the invention went well beyond propellers. And they found that using recirculating balls for linear motion was a complex problem. As design work shifted to the Ball Bushing bearing, the constant-speed propeller received less and less attention. In fact, the propeller work never went beyond prototype, but the Ball Bushing bearing never looked back. Interest in it prospered at the Thriftmaster Div., Thomson Industries, Long Island City, N.Y.

The first try at a rolling-element linear bearing had oval tracks in the shaft. One longitudinal section was shallower than the rest of each track, including the curved ends, so this section carried the load. A major drawback: limited stroke. To overcome it, the next step put the oval track in the bearing sleeve. It was hard to machine curved ends inside the sleeves, so sleeve ends were counterbored to accept inserts with the curved sections. The counterbored bearing’s oval track also had a return segment of greater clearance than the load-bearing segment.

These prototypes were exciting, but performance was poor and they were hard to make. Occasionally a track would recirculate, but more often the prototypes just didn’t work. It sank into the original team that this type of bearing required tight tolerances; design innovations were sorely needed.

The next prototypes tried to simplify manufacture with separate outer and inner sleeves. The outer sleeve carried load; the inner sleeve retained the balls. Small islands screwed into the oval tracks of the inner sleeve held the balls in place. Deep longitudinal ball return guides were machined to the same depth as the outer-sleeve counterbores. The return guides were now much deeper than the load-bearing tracks, so a gradual slope was needed to ramp balls into and out of the load-bearing region.

This prototype was still essentially carved from solid steel and was prohibitively costly to make. And yet it didn’t perform very reliably. Nevertheless, one advertisement was placed in a trade magazine to measure market potential. The response was a deluge; manufacturability became crucial.

The first practical result bloomed quickly in the 1950s as the standard Ball Bushing bearing, an all-steel unit with stamped steel retainers. Ball tracks were still oblong with single load tracks and single return tracks for each of several circuits. And they could hold alignment after installation because of low wear, but they weren’t inherently self-aligning.

The next major development came in the late 1960s and early 70s with what the manufacturer calls the “Super Ball Bushing bearing.” It offers 3 times the load capacity of the standard Ball Bushing bearing or 27 times the life. Plastic retainers replace steel retainers and individual bearing plates are machined separately. Each bearing plate has a single row of load-bearing balls. Manufacturability is better: The ball retainer is produced by inexpensive precision injection molding, and bearing plates are shaped by low-cost cold forming from bearing steel, then hardening to 60 Rc. Also, innovations by Mr. Magee in bearing plate shapes allow some self-alignment, that is, plate pitch as in Figure 2, though there is no ring. Moreover, ball-conforming grooves help capacity because of greater ball contact.

Other, later developments included the MultiTrac bearing, which put loadbearing tracks side by side without separate plates. In recent years several other prototypes, not commercially available, provided innovation seed for the newcomer — the Super Smart Ball Bushing bearing with its ability to handle misalignment in pitch, roll, and yaw.

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