Appliance of Linear Bearing in the Standing Vacuum Furnace

Abstract: This thesis introduces the appliance of linear bearing in the standing vacuum furnace, linear bearing restricts the movement of the screw to eliminate shaking of the workpiece during looding and unloading. The precision of design and fixing must be assured, on this condition, the designer must choose appropriate type of the linear bearing and verify the touching stress. The linear bearing is in order during practice run, and is proved that the calculation is logical.

Key words: linear bearing; precision; touching stress

0. Introduction

Vertical vacuum furnace refers to a vacuum heat treatment furnace in which the furnace body is vertically placed and the lower furnace cover is lifted to achieve material loading and unloading. The main body of the vacuum furnace (excluding peripheral systems such as vacuum acquisition system, water cooling system, pneumatic system, and gas cooling circulation system) includes the furnace body, heating chamber, furnace body support frame, lifting operation mechanism, and lifting guide mechanism. At present, a large part of vertical vacuum furnaces in China have unstable lifting operation of the furnace cover, mainly due to the uneven and smooth guiding plane of the guiding mechanism, and the large parallelism error between multiple guiding planes. The loading and unloading process of a vertical furnace requires stability and no noise, so great attention should be paid to the design of the guiding mechanism.

1. Structural characteristics and installation accuracy of the guiding mechanism

Figure 1 is the main structure diagram of the ZSL-300 vertical vacuum furnace designed for a certain research institute. In Figure 1, the lower furnace cover is driven up and down by a spiral lifting mechanism. The spiral lifting mechanism mainly consists of four sets of fully synchronized ball screws, which have both rotational and linear movements. The screw nut is installed in the box and only performs rotational movements.

The main components of the lifting guide mechanism described in this article are linear bearings, which are mainly composed of support rails, optical axes, and multiple sliders (as shown in Figure 2). The vertical furnace body is supported by four pillars, with four linear bearing support rails fixed on these four pillars. The optical axis is installed on the support rails, ensuring maximum strength and rigidity throughout the entire operation process. The end of the ball screw is fixed on the slider of the linear bearing, and the slider moves back and forth along the optical axis, which restricts the movement of the ball screw and makes the operation of the lower furnace cover more ideal.

Fig.1 The main structure diagram of the standing furniture

Due to the four sets of ball screws, four sets of linear bearings are equipped. The precision of manufacturing and installation determines whether the equipment will have good operating conditions. Due to the need for bearing installation accuracy, the parallelism error of the optical axis of the four sets of linear bearings does not exceed 0.1 mm. So the installation surfaces of the support rails on the four pillars supporting the furnace body must be machined surfaces. In addition, when the equipment is in place, ensure that the parallelism error between the four installation surfaces does not exceed 0.1 mm and the perpendicularity error with the horizontal plane does not exceed 0.1 mm. When assembling the screw and slider, it is also necessary to ensure that the parallelism error between the screw and the optical axis does not exceed 0.1 mm.

2. Force analysis and safety verification

The linear bearing shown in Figure 2 has a relatively uniform force on the balls inside the slider when used horizontally, resulting in a longer service life of the slider. When used in vertical vacuum furnaces, it is necessary to analyze the force situation of the slider in detail, and then determine its model based on the rated load of the linear bearing.

At the beginning of the force analysis, the weight of the entire load of the spiral elevator needs to be calculated, including the workpiece that needs heat treatment, furnace bed components (used to support the workpiece), lower heating chamber cover, lower furnace cover, and water in the lower furnace cover jacket. In the ZSL-300 vertical furnace, the total weight of the load is:

G total=14060 N (1)

There are four sets of ball screws and guide mechanisms, and the weight carried by each set of ball screws is:

G=G total/4=3515 N (2)

Fig.2 The structure diagram of the linear bearing

In Figure 3, the weight of the load is transmitted to the connecting block through the support block, simplifying the force to a point, which is the force point in the figure. The connecting block connects the ball screw and the linear bearing slider. The ball screw carries the weight, and the linear bearing constrains the movement. Perform force analysis on the connecting block, as shown in Figure 4. Before analyzing the force, it is important to clarify that the ball screw cannot be imagined as an ideal rigid body. If it is not constrained, it will swing during operation. So, the load weight and the lifting force of the ball screw form a torque M1 on the connecting block, and the function of the linear bearing is to balance it through a reverse torque M2, thereby achieving the effect of restraining motion. Due to the vertical use of linear bearings, the reaction force between the slider of the linear bearing and the connecting block is in a gradually changing load distribution state. That is to say, q in Figure 4 is not a fixed and unchanging quantity, but increases with the increase of the distance between the force surface G and N1 on the connecting block, until the lowest end of the slider ball reaches its maximum, which is a dangerous point of ball damage inside the slider. If the contact stress between the ball and the smooth bar at this point is calculated to be within the safe range of the bearing steel, and the load is within the safe load range of the linear bearing, then the selection of the linear bearing and the design of other parts and components are reasonable. In Figure 4, the distance between the action point where q reaches its maximum value and the action surfaces of G and N1 is L1:

L1=0.128 m (3)

The distance between the point of action of G and N1 is

L2: L2=0.025 m (4)

Based on the known conditions above, it can be concluded that:

M1=G ×  L2=3515 N × 0.025 m=87.875 N · m (5)

Also: M2=M1 (6)

The line of action of the load q on the connecting block, with the origin of the coordinate system at the intersection of the x-axis and the action surfaces of G and N1. The curve expression formed by non-uniform load in the coordinate system is:

y=Ax (7)

y is the non-uniform load q, in N/m; x is the distance, in meters; A is the slope of the curve, in N/m².

The total torque formed by non-uniform loads is M2, expressed as follows:

Substituting equations (3) and (7) into equation (8) yields:

From equations (5) and (6), it can be solved that:

Due to the radius of the balls inside the linear bearing slider:

The non-uniform load acting on the bottom ball can be simplified as a concentrated force N2 acting at the center of the ball, as follows:

From equations (3), (10), and (11), it can be concluded that:

N₂ = 125 N ( 13)

By calculating the maximum contact stress to verify the linear bearing, the contact type between the ball and the optical axis of the linear bearing is ball to cylinder contact, and its maximum contact stress is σ _max:

In the formula: σ_max is the maximum contact stress between the inner ball of the linear bearing slider and the optical axis of the linear bearing; n3 is the coefficient, taking n3=0.9942; E is the elastic modulus, E=206 ×  10⁹ Pa; R₂ is the radius of the optical axis of the linear bearing, R₂=0.02 m.

According to known conditions such as equations (11) and (13), it can be calculated that:

σ Max=2852 MPa (15)

The strength conditions for contact problems are:

According to equations (15) and (16), the strength condition is met.

Next, the safety of the selection will be verified by analyzing the maximum uniformly distributed load that the linear bearing can withstand. The rated dynamic load of linear bearings is:

C=2150 N (17)

So the maximum uniformly distributed load that linear bearings can withstand is:

[qmax]=C/L₁=16797 N/m (18)

The maximum value of the non-uniform load actually borne in this example is:

qmax=A · L₁=16090 N/m (19)

So: qmax<[qmax] (20)

This further verifies the safety of the selection of linear bearings.

In this example, it can be seen that the design of size L2 is crucial. L2 follows the principle of smaller is better, and actual conditions allow it to be set to L2=0. Therefore, M1=M2=0. When selecting linear bearings, only the constraint of unstable screw operation needs to be considered.

3. Conclusion

The linear bearing of ZSL-300 vertical furnace has been running for two years without any faults since the successful debugging of the equipment, fully demonstrating the rationality of the design.

2024 January 3rd Week VAFEM Product Recommendation:

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