Single-Direction Thrust Ball Bearings 51105 and 51107: A Practical Technical Overview

Among the many families of rolling bearings, the single-direction thrust ball bearing occupies a narrow but essential niche. It is the simplest, cheapest and most widely stocked solution whenever a machine must carry a purely axial load along a vertical or horizontal axis while the shaft turns. Two of the most frequently specified sizes in this family are 51105 and 51107. They look almost identical on a drawing, they share the same design philosophy and the same light-series proportions, yet they differ in ways that matter once load, speed and envelope are put on the table. This article explains what these two bearings are, how they behave, where they are used and what engineers should watch out for when choosing between them.

Reading the designation

Both part numbers follow the same logic. The leading "5" identifies the bearing as a thrust ball bearing with a flat seating surface, distinguishing it from radial ball bearings and from spherical or cylindrical thrust types. The following "11" is the dimension series: an extra-light thrust series, which means the bearing is deliberately thin in relation to its diameter in order to save axial space. The final two digits encode the bore size, where the number is multiplied by five to give millimetres. The "05" therefore corresponds to a 25 mm bore, and the "07" to a 35 mm bore. A designer who understands this convention can read a whole catalogue page at a glance, because every 51 series bearing shares the same proportional relationship between bore, outside diameter and height.

Construction and load path

A single-direction thrust ball bearing consists of three elements: a shaft washer, a housing washer and a ball-and-cage assembly between them. The two washers are hardened, ground and lapped to a high degree of flatness, and each carries a single circular raceway groove. The balls are held in a sheet-metal cage that keeps them evenly spaced and prevents them from rubbing against one another. Because the raceways are flat annular grooves rather than deep conforming channels, the contact angle is essentially 90 degrees: the bearing is designed to accept force along its axis and nothing else.

This is the single most important characteristic of the type. A 51105 or 51107 will tolerate only a very small incidental radial force, and it has no capacity at all to resist reversing axial load. The load must travel in one direction only, entering through one washer, passing through the balls and leaving through the other. If the shaft is pushed the other way — by vibration, by a reversing actuator, or simply by a load whose direction is uncertain — the bearing separates and the assembly loses its location. In such cases two bearings mounted in opposition, or a double-direction type, are the correct answer.

Because the balls run on flat washers, the load is spread over a comparatively large contact area and the bearing develops a high static capacity for its size. At the same time, the absence of a conforming groove means the balls cannot roll on a well-defined track: they slide a little, and they rely on centrifugal effects and lubrication to keep them from skidding. This explains both the type's excellent static strength and its modest speed ceiling.

The two sizes in numbers

Placed side by side, the difference is one of scale. The 51105 has a 25 mm bore, a 42 mm outside diameter and an 11 mm height, with a minimum chamfer of 0.6 mm. Its basic dynamic load rating is around 18 kN and its basic static load rating is around 39 kN. The 51107 has a 35 mm bore, a 52 mm outside diameter and a 12 mm height, again with a 0.6 mm chamfer. Its dynamic rating is approximately 20 kN and its static rating approximately 51 kN.

The step from one to the other is worth noting. Going up by 10 mm of bore adds roughly 11 percent to the dynamic capacity but nearly 30 percent to the static capacity, while the height grows by only one millimetre. That is typical of thrust bearings: axial capacity rises faster with diameter than with height, because the raceway circumference grows. The practical consequence is that a designer who is short of axial space but has room radially will often jump to the larger size rather than try to gain capacity by other means. Published figures differ slightly between manufacturers, as do the exact reference speeds, so the values above should be treated as representative rather than absolute.

Load rating logic

For thrust ball bearings the static rating usually governs the selection, not the dynamic one. Rotating machines rarely fail these bearings by fatigue; they fail them by brinelling the raceway when a shock load, a heavy static load or a poorly controlled preload flattens the track. A static safety factor — the ratio of the basic static rating to the actual maximum axial load — of two or three is normal practice for smooth running conditions, and considerably higher for applications with shock, vibration or frequent standstill under load. Applying that logic, a 51105 comfortably carries a continuous axial load in the region of ten to fifteen kilonewtons, and a 51107 in the region of fifteen to twenty-five, provided the load is steady, correctly centred and adequately lubricated. Off-centre loading, which produces a tilting moment the type cannot resist, must always be avoided or accommodated by other means.

Speed

Speed is the other decisive variable. Thrust ball bearings are intrinsically slower than radial ball bearings of similar size, because the balls must be dragged around by friction rather than driven by a conforming groove. Reference speeds in the order of six thousand revolutions per minute for the 51105 and five and a half thousand for the 51107 are typical when oil lubrication is used, and grease lubrication reduces those figures by roughly a quarter to a third. Beyond the recommended range, ball skidding, cage stress and heat generation rise quickly, and the bearing may fail long before its calculated fatigue life is reached.

Mounting and abutment details

Correct assembly matters more here than in most bearing applications. The shaft washer, whose bore matches the shaft, rotates with the shaft and must be butted squarely against a shoulder. The housing washer has a larger bore — around 37 mm in the 51107 — so that it clears the shaft entirely and can remain stationary in its housing without rubbing. Reversing the two washers is a classic assembly error: the bearing may appear to fit, but the load path, the contact geometry and the clearance are all wrong, and failure follows quickly. Both washers must be seated on flat, rigid, adequately supported surfaces; any distortion of the housing or shoulder is transferred directly into the raceway and destroys the flatness on which the bearing depends. Minimum axial load also deserves attention, since a lightly loaded thrust bearing at speed will skid rather than roll.

Lubrication and care

Lubrication should be generous. Oil bath or circulating oil is preferred for continuous duty and higher speeds; grease is acceptable for moderate speeds and sealed, low-maintenance arrangements. Because the type is normally supplied open, protection against dust and coolant ingress depends entirely on the surrounding machine design, and contamination is one of the most common causes of premature failure. Periodic inspection of the washers for brinelling, wear tracks and discolouration gives a reliable picture of whether the bearing is being loaded, aligned and lubricated as intended.

Typical applications

The 51105, being compact and inexpensive, appears in small gearboxes, vertical pump shafts, valve actuators, screw jacks, clutch release mechanisms, indexing tables and light lifting equipment. The 51107, with its extra capacity, is found in larger worm reducers, machine-tool lead screws, hoist and crane hooks, hydraulic pump shafts and vertically mounted electric motors. In both cases the bearing does one job: it holds an axial position and lets the shaft turn with minimum friction, in one direction, within a defined envelope.

Choosing between them

Choose the 51105 when bore size and radial space are the binding constraints and the axial load is modest. Choose the 51107 when the load is heavier, when shock is present, or when a longer service life is wanted at the same nominal load — accepting 10 mm more bore and a slightly larger housing. In either case, verify that the load is purely axial, that it never reverses, that the washers are correctly oriented and seated flat, that the speed stays within the lubricant's capability, and that the static safety factor reflects the real duty. Get those five points right and these two small, inexpensive bearings will quietly outlast much of the machinery around them.