May 2011 Archives

Here, the differences of Linear Bushing shapes (straight and flanged types) , applications, and mounting cautions.

(1) Straight and Flanged Linear Bushings

[Photo 1] is the Straight Type, and [Photo 2] is the Flanged type. The Flanged Type [Photo 2] has the following advantages.

[Fig.1] is a graphical explanation of a MISUMI Flanged Linear Bushing. Adding a flange on a straight bushing will increase the housing diameter and length, but the flanged linear bushing is compact since the bushing and an outer cylinder are integrated into one structure. A compact design is possible while retaining load bearing capacity.

(2) Usage Differentiation of Straight Type and Flanged Type

Following are the points to consider when selecting Straight or Flanged bushings.

Linear bushings can be used as the moving structure or as stationary bearings where the shafts move. [Fig.2] below is a multi-axes X-Y-Z-θ stage, and each axis can be analyzed as follows.

In the illustrated case, the X-axis moving linear bushings on section a) will need to be mounted securely since they bear the inertial forces of the moving mass. On the section b), the housing mounted linear bushings only receive the reaction forces of the friction from an air cylinder driven shafts. Therefore, a compact design using the Straight Bushings is adopted. Furthermore, the opposing layout of dual shafts against the rotary moments of the θ axis enables the use of Straight Bushings with sufficient rigidity. The c) section can be considered as the same as the b) section in terms of the shaft moving direction, and no large forces apply.

(3) Installation of Linear Bushings and Notable Points

(1) Installation of Linear Bushings

Straight Linear Bushings are mounted/secured using Retaining Rings or Fixing Plates. (See [Photo 3] & [Fig.3])

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(2) Notes on Linear Bushing Mounting Angle

Linear Bushings have varying number of ball rows depending on types and shaft diameters. They typically have 4~6 rows of ball circuits and are placed in equally spaced angles. In horizontal motion applications, placing a ball row at 12 o'clock position (left illustration in [Fig.3]) should be avoided since the balls would have to bear concentrated loads.

In the [Fig.4] with 5 ball row type, the load capacity ratio (Right divided by Left) would become as shown below. Therefore, it is recommended to mount the bushings as shown in the illustration on the Right.

Static Load Rating (Right divided by Left) = 1.46

Dynamic Load Ratin (Right divided by Left) = 1.19

Linear guiding devices are the most frequently used motion elements on automation equipment for palletizing, transferring, positioning and assembling. Here, linear bushing application tips will be explained along comparisons of [1] Linear Bushings, [2] Linear Guides, and [3] Plain Bushings.

(1) Characteristic Comparisons of Linear Bearings

Three linear guiding types are roughly compared below.

Type Load Capacity Friction Coefficient Guide Accuracy Environmental Resistance Maintain ability Cost Linear Bushings Low Linear Guides High Plain Bushings Medium

Relationship of the characteristics above and the structures are explained below.

(2) Relationship of the characteristics and the structures of linear guidings

(1) Differences in Load capacities

Linear Bushings vs. Plain Bushings

a）

Linear motion units with linear bushings and plain bushings have suspended guide shafts supported on the ends and the shafts would deform under high loadings ([Photo 1]). (For vertical motion applications where the load do not apply horizontally on the shaft, this problem does not exist and a compact design is possible)

Linear Guides

b）

Excellent load capacity due to the rails directly mounted on bases.

(2) About Friction Coefficient Differences

These differences are due to the differences in sliding methods (rolling motion or sliding motion). These differences will directly affect the selection of driving actuator.

	a）	Low Friction Resistance = Little Fiction force = Possible to drive with small motors = Rotary motion to Linear motion Conversion
	b）	High Friction Resistance = Large Fiction force = Requires large torque/force = Push directly with linear cylinder

Cautionary Points on Usage

	1.	The high friction coefficients affect driving actuator selections and operation heat generation. The plain bushings are not suitable for frequent motion at high speeds.
	2.	Air cylinders are not well controllable for acceleration/deceleration velocities thus some soft-stop mechanisms such as shock absorbers will need to be installed to achieve high speed operations and vibration control.

(3) Differences in Guide Accuracies

This parameter is mainly determined by clearances between the rails and bearings.

	a）	Linear bushings are used with round shafts, and operate with small amount of clearances such as "Clearance Fit: g6" or "Medium Fit: h5"
	b）	Linear guides are used as pairs of dedicated rails and precision bearings in "Slight Clearance (0~3 μm) or "Preloaded (-3~0μm).
	c）	Guiding accuracies of plain bushings are lower since they are used with shaft clearances larger than linear bushings.

Cautionary Points on Usage

Bearing contact arrangements of linear bushings and linear guides are different. Linear bushings have Point Contact arrangement and localized contact pressures are high. Linear guides use rails with concave grooves that are shaped to fit the spherical shape of the bearing balls and the contact arrangement is a Surface Contact. Applied loads are distributed over the contact areas. The contact arrangement differences reflect on the load capacities of these systems ([Fig.1] & [Fig.2]).

(4) Environmental Resistance Characteristics and Maintainability

This is determined by the construction material.

	a）	Linear bushings and linear guides depend on lubrications used for long term reliability thus they cannot be used in environments exceeding the environmental resistance of the lubrications used.
	b）	Plain bushings can be used without any lubrication and has higher environmental adaptability and better maintainability.