The Importance Of Bearing Preload And Axial Adjustment

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Preloading and axial adjustment is a vital but still largely misunderstood area of bearing technology. However, when applied correctly, preloading can reduce or overcome the causes of bearing vibration, heat build-up, noise and fatigue. Gary Hughes, Product Engineering Manager at The Barden Corporation, describes the pros and cons of the three primary methods of applying preload to a bearing. Preloading and axial adjustment is a method of removing or controlling the internal clearance of a bearing. This is important because the degree of internal clearance within a bearing can influence a variety of factors, including noise, vibration, heat build-up and fatigue life. When applied correctly, preloading reduces or overcomes the causes of these problems: it controls radial and axial play; provides predictable system rigidity; reduces non-repetitive run-out; reduces the difference in contact angles between the inner and outer rings at very high speeds; and controls ball skidding under very high acceleration. Bearing Yield Axial yield is the axial deflection between inner and outer rings after end play is removed and a working load or preload is applied. It results from elastic deformation of balls and raceways under thrust loading. Similarly, radial yield is the radial deflection caused by radial loading. Both types of yield are governed by the internal design of the bearing, the contact angle and load characteristics (magnitude and direction). In all bearing arrangements where preload is deemed to be necessary, it should be applied as lightly as possible in order to achieve the desired result, avoiding excessive heat generation, which reduces speed capability and bearing life. In addition, it is important to select the most appropriate method of preloading, of which there are three main types: springs, axial adjustment, and preload ground Duplex bearings. Springs Of the three preloading methods, springs are usually regarded as the simplest and should be considered first. Spring preloading provides a relatively constant preload because it is less sensitive to differential thermal expansion than rigid preloading, and better accommodates any minor misalignment. Also, it is possible to use bearings that have not had the added cost of preload grinding. One disadvantage of using springs is that they cannot generally accept reversing thrust loads. Space must also be provided to accommodate both the springs and spring travel. In addition, springs may tend to misalign the ring being loaded due to the required clearance fits. Despite the disadvantages of spring preloading, this method is still a popular one, illustrated by the fact that there are numerous types of springs available for preloading, including coil springs, Belleville, wave or finger spring washers. With regard to mounting, the spring is normally applied to the non-rotating part of the bearing, typically the outer ring. This ring must have a clearance fit in the housing at all temperatures to ensure the preload force from the spring is effective. Axial Adjustment Another method of achieving internal clearance control is axial adjustment. This technique calls for the mounting of at least two bearings in opposition, so that inner and outer rings of each bearing are offset axially. Threaded members, shims and spacers are typically ways of providing rigid preloads via axial adjustment. This technique requires great care and accuracy in order to avoid excessive take up of internal clearance, which may occur during setup by overloading the bearings, or during operation due to thermal expansion. Precision lapped shims are normally preferred to threaded members here, as helical threads can lead to misalignment. The shims should be manufactured to parallelism tolerances equal to those of the bearings, because they must be capable of spacing the bearings to accuracies of 1-2 micrometres or better. Bearing ring faces must be well aligned and solidly seated and there must be extreme cleanliness during assembly. Axial adjustment does not increase bearing friction and is therefore preferred for very low torque applications. Duplex Bearings The third main method of applying preload is to utilise Duplex bearings. In contrast to springs and the axial adjustment method, using Duplex bearings offers the advantage that the means of achieving preload is built-in. In effect, this method of preload can be the simplest for the customer, who receives the bearings ready to mount and with the confidence that those bearings are preloaded to the precise requirement of the application. Duplex bearings are matched pairs of bearings that have their inner or outer ring faces selectively relieved by a precise amount known as the preload offset. When the bearings are clamped together during installation, the offset faces meet, establishing a permanent preload in the bearing set. Duplex bearings are normally speed-limited due to heat generated by this rigid preload. Duplexing is used mainly where the requirement is for predictable radial and axial rigidity. Duplex bearings can withstand bi-directional thrust loads or heavy uni-directional thrust loads. Other advantages include their ease of assembly and minimum runout. When using Duplex bearings, consideration should be given to the following: increased torque; reduced speed capacity; sensitivity to differential thermal expansion; susceptibility to gross torque variations due to misalignment; and poor adaptability to interference fitting. Most Barden deep groove and angular contact bearings are available in universal Duplex sets or can be furnished in specific DB (back-to-back), DF (face-to-face) or DT (tandem) configurations. DB mounting is suited to most applications that have good alignment of bearing housings and shafts. It is also preferable where high moment rigidity is required, and where the shaft runs warmer than the housing. In contrast, DF mounting is used in only a few applications, primarily where misalignment must be accommodated. Speed capability is normally lower than a DB pair of identical preload. DT mounting employs tandem pairs that offer greater capacity without increasing bearing size, through load sharing. The DT pairs can counter heavy thrust loads from one direction but they cannot take reversing loads as DB and DF pairs can. DT sets do not have inbuilt preloading and should be used in conjunction with spring preloading.

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It's going to be really interesting to test my Clausing 4902 with there MT 4 bar I bought. It's a plain bearing machine. I know from my level that the center part of the bed is worn down about 0.003" but I never had a good way to test the straightness of the prism or the headstock alignment.

My first project on it was a bar with a series of collars at 1" spacing which I attempted to turn separately to a tenth. It taught me a hard lesson about thermal expansion and the need for flood coolant. I hope to succeed in a rematch.

My first lathe was a Sears 109 which is a miniature 1880's machine. I still have it for the simple reason it was more trouble to sell than it's worth. But I'm beginning to develop a perverse fascination with trying to make some tight fit parts on it using nothing but spring calipers. The main job I did on it was rebush the leadscrew. That came off well, but I feel as if I cheated by using modern measuring tools.

I enjoy doing old school bench work with files and can readily hit parallel and square on 2" x 4" plates of 16 gauge steel to a thou using spring calipers and a square. For lots of work I can do it with files faster than I can set up the mill. So trying my hand at late 19th century machining seems an interesting way to spend an afternoon. Modern machines were built by people using such machines and methods. I admire skill in any domain and have spent my entire life trying to learn as many as I could. The fly in the ointment though is that they are transient and I have learned the hard way that things I was very proficient at 30 years ago I can no longer do without significant refresher practice.

120 years ago if a ship broke down at sea everyone died if the engineer couldn't fix it. So a marine engineering school exercise back then was to take a round bar and flat plate, file the bar hexagonal and make a hex hole in the plate so that the bar was a close sliding fit in all 6 positions for the length of the bar. I still haven't worked up the courage for that. But maybe some day.

I completely agree that generally you get what you pay for. However, I've been pretty lucky. I spotted my bar in the spindle and it had good contact all around. The last inch or so at the tailstock end doesn't seem very good, but until I get the tailstock properly aligned it's hard to tell. In the meantime I just don't use the last inch or so. Similarly I found <0.0001" TIR on both a backplate mount ER32 chuck and an MT 2 version of Chinese origin. The collets are another story. The ones I've measured have 0.0005" TIR.It's going to be really interesting to test my Clausing 4902 with there MT 4 bar I bought. It's a plain bearing machine. I know from my level that the center part of the bed is worn down about 0.003" but I never had a good way to test the straightness of the prism or the headstock alignment.My first project on it was a bar with a series of collars at 1" spacing which I attempted to turn separately to a tenth. It taught me a hard lesson about thermal expansion and the need for flood coolant. I hope to succeed in a rematch.My first lathe was a Sears 109 which is a miniature 1880's machine. I still have it for the simple reason it was more trouble to sell than it's worth. But I'm beginning to develop a perverse fascination with trying to make some tight fit parts on it using nothing but spring calipers. The main job I did on it was rebush the leadscrew. That came off well, but I feel as if I cheated by using modern measuring tools.I enjoy doing old school bench work with files and can readily hit parallel and square on 2" x 4" plates of 16 gauge steel to a thou using spring calipers and a square. For lots of work I can do it with files faster than I can set up the mill. So trying my hand at late 19th century machining seems an interesting way to spend an afternoon. Modern machines were built by people using such machines and methods. I admire skill in any domain and have spent my entire life trying to learn as many as I could. The fly in the ointment though is that they are transient and I have learned the hard way that things I was very proficient at 30 years ago I can no longer do without significant refresher practice.120 years ago if a ship broke down at sea everyone died if the engineer couldn't fix it. So a marine engineering school exercise back then was to take a round bar and flat plate, file the bar hexagonal and make a hex hole in the plate so that the bar was a close sliding fit in all 6 positions for the length of the bar. I still haven't worked up the courage for that. But maybe some day. @michiganbuck I followed the same practice on VWs, but I follow the factory manual on my Toyota pickup which uses friction as the spec. After today's exercise I'm likely to play around with a scrap spindle and housing to get a better feel for how the end play and friction methods relate. I suspect that the change is more related to the demands of factory production than anything else.

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