December 2016 Archives

Terminology related to ball screw accuracy is explained in this table.

[Fig.2] is an example of inspection data for precision ball screws. The solid line represents measured data.

Description of inspection data

• Cumulative reference of this ball screw (target value) = 0.0 µm --- Setting value
• The green arrow [1] is the actual mean lead deviation = -15 µm (data readings)
• The orange arrow [2] is the variation -a) <variation for the effective thread length> = 6.0 µm (data readings)
• The red arrow [3] is the variation -b) <variation for any 300 mm interval> = 4.0 µm (data readings)

JIS B 1192 defines ball screw accuracy of the precision ball screws. This section explains the accuracy and grade of ball screws.

The major factor in determining ball screw accuracy is called "lead accuracy". (See the Note* for details.) The two evaluation items of "lead accuracy" and "appearance" are called "ball screw grades" and used for the classification of quality.

Notes*

= JIS =

= ISO3408-1: 1991 =

A standard ball screw is comprised of the following components:

Compared to slide guides, this structure is probably easier to understand. A slide guide is a unit that produces linear motion along the rail. On the other hand, a ball screw is a unit where screw shaft rotation causes a nut to travel in a straight line.

The increasing demand for making the device faster, more accurate, more durable and more reliable has resulted in the rapid spread of ball screws. [Fig.1] shows the "Amount of Sales and Trend in Shipment of Ball Screws, January 2006" published by Japan Machine Tool Builders' Association. You can see from this graph that the amount of sales and quantity has almost doubled in the two-year period. From #266, we will look at the ball screws from the basics to advanced knowledge.

(1)History of ball screws

In this volume, we will look at a case example of a "double-lever" created by connecting levers utilized in the boost mechanism. The double-lever method not only delivers the boost action in the simple seesaw-type principle but also allows the force direction to be changed.

[Fig.1] - a) illustrates the boost mechanism for the double-lever method. The amplified force (F) is determined by the relationship between the arm length ratio and the equilibrium of forces.

[Fig.1] - b) is a simple fixture (tool) called a "block holder", utilizing the double-lever method. This mechanism is often adopted for tools designed to carry heavy square-shaped items like bricks and blocks.

Although the seesaw-type boost allows the operation of heavy objects with less of force, it is important to remember that the principle of the workload <Workload (Kgm) = Force (Kg) × Movement (m)> always applies. Less amount of force means more movement. If manpower is required for larger strokes or reducing a particular task cycle (such as moving or bending) does not make the entire task inefficient, it is possible to amplify the forces or movement based on this workload principle.

Application examples

1. Workpiece (block) conveyance fixtures
2. Washing and Rinsing clamp
3. Conveyance clamp in heat-treating furnace