**2. Ovality measuring on production plant**

42 Heat Treatment – Conventional and Novel Applications

are discussed in [ZVL & ZKL, 1996; ZVL, 2008].

requirements for ovality of inner rings (figure 2).

**Figure 1.** Tapered roller bearing

**Figure 2.** Inner ring of Tapered roller bearing

rings.

modify the outer ring, inner ring and rolling elements. Mentioned company is considered to be a "Mercedes" in the field of production of tractors, harvesters and similar equipment. The demands on parts are therefore accordingly high. This article will not discuss all the modifications necessary to adjust the technological process of production in order to meet customer's requirements, although they are all interrelated, influence each other and represent a desired outcome of the targeted modification. We will deal only with the parameter that can be affected by heat treatment of the material in order to avoid unwanted deformations in the material and comply with a stricter requirement on ovality for inner

Papers of authors [Jech, 1983; Panda et al., 2011; Vasilko, 1998] address processes of heat treatment of bearing components with related checks and tests (dimensional, chemical, metrological and endurance tests) and technological aspects of production of bearing rings

The goal of the work was to modify the tempering process to eliminate or preclude undesirable deformations of the material and to ensure compliance with the stricter Technologically, ovality results from non-symmetrical distribution of internal tensions before hardening and uneven heating and cooling. Ovality is a certain type of circularity deviation.

Since 1950s, devices using very accurate rotational tables or spindles are used in modern industry to measure circularity. The measuring base in this method is the axis of the component to be measured. The measuring device ensures very high accuracy, often better than 0.1 μm. However, it requires time-consuming preparations including centring and alignment of the component, therefore it is intended rather for laboratory measurements. These measuring devices are rarely used in production, also due to their low measuring capacity.

Ovality is checked at the output from the heat treatment line. A special diameter gauge (Figure 3) that included a dial deviation meter was used to measure ovality (table 1). Ovality was measured on three rings so that an ovality deviation was recorded after each rotation by 10°. The following procedure was applied. An bearing ring was inserted into the gauge. The ring was rotated manually in 10° increments. A value was read from the deviation meter after each rotation. If the deviation was higher than 0.2 mm, the ring being measured was discarded. It did not meet quality criteria and could not be passed on for subsequent hard machining.

A high deviation in ovality may indicate uneven heating or cooling during heat treatment. Ovality measurement was made on three inner rings selected randomly from different batches. Ring No. 1 shows acceptable ovality up to 0.2 mm in maximum. Subsequent grinding operations can reduce or remove this deviation from circularity. Ring No. 2 shows ovality over 0.2 mm, so it is not suitable for further machining. It must be discarded. The lowest ovality value can be seen in ring No.3. For ring No. 2, it is necessary to identify the cause of such a significant deviation in cylindricality. The whole batch the ring No. 2 comes from should be checked.


**Table 1.** Measured values of ovality


**Table 2.** Measured values of ovality - continuation

Deformation Reduction of Bearing Rings by Modification of Heat Treating 45

**Standard** 

**32010AX Special**

**Bearing part and parameter 32010AX**


Material deformations and ring ovality are caused by internal tension generated during machining and heat treatment operations. To process a bearing ring by turning, it has to be fixed at three points. The fixing is done pneumatically. A deformation may occur due to poor fixing, or due to a failure to follow technological conditions, when more material is removed. When rings are grinded after heat treatment, similar undesired deformations occur, if technological conditions are not followed. Major deformations occur even during the heat treatment itself, i.e. when the rings are hardened, due to uneven heating and cooling. Deformation that appeared after heat treatment are then reproduced at subsequent

The customer accepted only 0.003 mm of stricter ovality for the outer ring after grinding, compared to the standard prescribed value of 0.006 mm (see Table 4), which tightens the requirements by 50%. To achieve this final ovality of rings after grinding , then the ovality of rings after hardening can be no more than 0.1 mm, which is also a stricter value compared to

Ovality is defined as the difference in diameters measured in one plane perpendicular to each other. This means that, for example, the maximum diameter Dmax is measured first – i.e. the maximum value is found when the ring is turned, then the ring is rotated by 90 ° and the second, minimum diameter Dmin is measured. The outer shape of the ring should be close to a circle, but in fact, the outer ring is elliptical in shape. Our aim is to keep this ovality as small as possible.

**Inner ring**

**4. Experimental section** 

grinding, worsening this effect even more.

the standard requirement of 0.2 mm.

**Figure 4.** Definition of ovality

**Figure 3.** The principle of measuring ovality of outer shape of the outer ring of a roller bearing. 1 - dial deviation meter, 2 - outer ring, 3 - moving measuring contact line, 4 - supporting parts


**Table 3.** Measured values of ovality - continuation

## **3. Customer's requirements**

In principle, the customer required a bearing that would withstand higher axial loads without any major run-in and with reduced ring ovality due to the placement of the rotor in an axial piston hydroelectric generating set. Table 4 shows a comparison of basic modified parameters between the standard design (Standard) and the design required by the customer (Special). As mentioned before, this paper will discuss only the issue of reducing ovality in the inner ring. Finding a solution for such task is even more difficult because the bearing is a thin-walled bearing (AX series) that is much more sensitive to material deformations and ring ovality compared to other bearings with a more favourable ratio of ring thickness and width.


**Table 4.** Comparison of basic parameters of 32010AX bearings between "Standard" and "Special" designs
