Push/Pull Method for Preloading 2-TS Spindles: Why Preload?

Spindles designed for machine tool applications, such as milling, grinding and turning, require the bearings to be set in preload to meet speed, stiffness and accuracy requirements. More importantly, the workpiece cut finish needs to be visually free from chatter and meet surface roughness requirements. A method of preloading the spindle accurately and consistently is required to meet the industry demand for machine tool spindles.

Achieving an accurate mounted preload setting for a spindle with two single row Tapered Roller Bearings (2-TS) can be using a procedure known as the push/pull method where a series of axial loads are applied in both directions to a spindle assembled in endplay and the axial displacement is measured for each applied load. The exact endplay is determined from the force vs. deflection plot, then the measured endplay is used to calculate the thickness of a spacer needed to achieve the target mounted preload.

Before getting into the details of the push/pull procedure, it is necessary to cover tapered roller bearing  and spindle basics including the load zone, preload and stiffness, and why preload is a requirement for milling, grinding and turning. Understanding these basic concepts will put the push/pull method into context and greatly help the reader to understand the procedure as described in this paper.

Single-row tapered roller bearing (TS). The basic tapered roller bearing (TRB) is the TS bearing. “T” refers to tapered and the “S” indicates that it’s a single-row bearing. TS bearings usually are used in opposed pairs on a shaft and that will be discussed in the next section. The TS bearing consists of a cone (inner ring), cup (outer ring), rollers, and a cage. (See Figure 1.)

TS bearing features and geometry. Figure 2 highlights key bearing features referred to later in this paper. Bearing stand is the deviation from nominal of the assembled width across the bearing from the cup backface to the cone backface. It is important when determining the axial stack-up of the assembled spindle. Because a TRB is asymmetrical, it possesses four unique ring faces: the cup front and back faces and the cone front and back faces. The backfaces can be thought of as thrust faces while the front faces as non-thrust faces. The rib is important because when load is applied to the bearing, the roller large end seats against rib.

The apex is the point where lines parallel to the contact (inner and outer raceways) surfaces meet. Rollers move around the races in a perfect circle (true rolling motion) that means that there is no skidding or slipping between the surfaces. Preload ensures that rollers are properly seated between both raceways. (See Figure 3.)

2-TS spindle. The 2-TS IM indirect mounted arrangement is the most commonly used TRB spindle arrangement because it is thermally stable. A thermally stable spindle is a spindle where the decrease in preload due to the shaft growing in length faster than the housing is offset by the increase in preload due to the cones running hotter than the cups. The setting adjustment is accomplished with a locknut against the cone backface . (See Figure 4a.)  The 2-TS DM direct mounted arrangement has a shorter effective spread and has a setting that the 2-TS IM spindle and is more susceptible to a thermal runaway due to the increase in preload due to the shaft growing in length faster than the housing is offset by the increase in preload due to the cones running hotter than the cups. The setting adjustment is accomplished with the housing endcap pilot against the cup backface. (See Figure 4b.)

The load zone. The load zone is the percentage of rollers in contact with both raceways expressed in degrees (0° to 360°). As an example, if a bearing with 18 rollers has 12 rollers in contact both raceways, the load zone would be 240°. The load zone is a function of the bearing’s mounted setting. Bearings with mounted endplay (internal clearance) have load zones less than 180°. Bearings with zero mounted endplay (clearance) have a load zone of 180°, bearings set with a light preload have load zones between 180° and 360°. A mounted setting that yields a load zone of 360° is considered a heavy preload. (See Figure 5.) Most machine tool 2-TS spindles are set with a heavy preload.

Bearing stiffness. When the bearing reacts to an external radial load, like that shown in Figure 6, the load is transmitted from the inner ring (shaft) to outer ring (housing) or vice versa via the rollers.

The rollers in contact with both races react to the load. The more rollers in contact with both raceways, the lower the load per roller and hence a smaller radial deflection between the cone (shaft) and cup (housing). The radial stiffness is defined as the applied radial load divided by the radial deflection of the bearing. The stiffness will also vary with the applied load.  (See Figure 7.)

Preload. The one common denominator for all machine tool spindles is that the bearings need to be set in preload. Accuracy, stiffness and speed are all critical attributes for machine tool spindles, preload is how these attributes are controlled by the assembler.

Accuracy. Machine tool spindles, regardless of speed, cannot tolerate endplay because of accuracy requirements. A typical HSK-100 milling spindle is only allowed a total runout of 5μm, endplay can easily consume this budget and much more. Endplay would be unacceptable in an application where accuracy is critical.

Stiffness. Stiffness as we discussed in the previous section is a critical characteristic of all machine tool spindles whether the spindle is milling, grinding or turning. Machine tool builders all have stringent cut quality (visual appearance and surface roughness) requirements. Spindles that do not have enough preload can result in chatter and waviness on the workpiece surfaces. 

Speed. High speeds are required in many machine tool applications. When speeds begin to approach 10 meters per second of rib speed (see Figure 8) rollers that are not properly seated between both raceways under high speeds can skew and slide. These conditions can cause heat and surface initiated damage that can significantly reduce the spindle service life. Preload helps ensure that all the rollers are properly seated between both raceways.

It is important to mention that the optimal setting also depends on the type of arrangement (2TD-IM versus 2TS-DM) and the assumed shaft/cone and housing/cup temperature assumptions.

This report continues... Read Push/Pull Method for Preloading 2-TS Spindles: Achieving Preload
Eric Faust is an Application Engineering Specialist with The Timken Company. Contact him at [email protected]