March 2012 Archives

(1) What is a clean room

A clean room is a room supplied with particle removed air filtered by high performance filters (HEPA filter, ULPA filter). The room air pressure is kept high (positive) to prevent infiltration of airborne particle contaminants.

(2) Types of clean rooms

There are following 3 types of clean rooms classified by air flow types (see [Fig.1])

(3) About cleanliness localization

Cleanliness localization is a concept of keeping only the required localized space clean where especially high levels of contamination control within a clean room. Following two types are the main elements of cleanliness localization, and represent a concept of realizing clean environments where necessary as opposed to making the entire room clean.

(4) Standard clean room example

[Fig.2] shows an example of a standard clean room. Clean rooms are design to administer proper contamination control for the work to be performed. The cleanliness is made to get higher from the entrance on. The layout is designed according to the types of work (work area, equipment area, maintenance area, passage way, preparation room, etc.).

With the domestic industrial trends shifting towards high added values and high quality over ppm order, demands for highly functional equipment cleanliness with low investments are increasing.

(1) Technology elements required for clean compatible designs

Principles of creating and maintaining clean environments are the following 3.

1. Do not bring in contamination.
2. Do not generate contamination.
3. Do not allow contamination to accumulate.

The elemental technologies to form and maintain each one of the principles on LCA mechanisms would be as follows.

[Table 1] Main technological elements comprising the 3 principles of cleanliness

Procurement of a linear motion mechanism with a servo motor will require the following man-hour steps of a design engineer.

1. Requires many design process man-hours due to high number of components involved.
2. Long lead-times for precision construction elements such as linear guides and ball screws.
3. After the parts are procured, accuracy oriented assembly work will be required.

Therefore, some challenges exist through the process of designing to completion and long lead-times. The new product "Single Axis Unit" is a proposal to assist solving these issues, and has the following characteristics.

(1) Characteristics of "Single Axis Unit"

Component procurement process and assembly work can be omitted with the following characteristics.

1. Ball screw driven units compatible with various servo motors.
2. Ball screw DIA., lead/base width/stroke/table length are selectable.
3. Short delivery time, specified with only part numbers (8th day shipment).

(2) Supplementary explanation for usage

Invention of rollers contributed in dramatically reducing friction resistance when transporting un-suspended objects. Rather than dragging a stone over strewn sand, the stone can be moved with much less force if pulled over rollers. The former is an equivalent of slide linear bearing arrangement and the latter is the roller linear bearing arrangement. Ball screws are mechanical components utilizing this roller linear bearing principle to convert rotary motion into linear motion and vise versa, and have the following characteristics.

Characteristics and benefits of ball screws

(1) Very small rolling friction coefficient. (Friction coefficient = 0.002 ~ 0.004)

• High energy efficiency obtained due to very small friction losses.
• Both "rotary to linear" and "linear to rotary" motion conversions are possible.
• Static friction coefficient and dynamic friction coefficient are very small, virtually no stick-slip occurs.
• High rigidity and low backlash can be made possible by applying preloads.

• Running life can be approximated by calculations.
• Quality and stability controlled by JIS Standards.
• Performance can be maintained by normal maintenance processes such as grease applications.

Characteristics and shortcomings

(a) Precision assembled unit with point contact sliding mechanism.

When two parts are fastened with a bolt, tension and compression forces are in a balance where the bolt is elongated and the fastened part material is compressed. If an external forces such as shock loads and vibrations apply to this fastening force balanced condition of the two parts, the bolt may become loose. This bolt loosening may result in equipment damages, defective product production, and critical defects that may lead to accidents. Here, typical mechanisms of screw loosening and countermeasures will be explained.

(1) Screw loosening

• A bolt fastening two parts may become loose when the following a) and b) simultaneously occur (see [Fig.1]).
  • There are contacting surfaces 1~4 of the two parts being fastened, and external forces are applied...
  • There is a clearance between the male and female threads at nut contact 1, and a relative slippage occurs.
• The external forces that affect the bolt loosening are: 4 types shown in [Fig.1] a)~d). These external forces cause "Reverse rotation motion" on the bolt resulting in loosening (see lower rows of [Table 1]).
  • Axial direction external force
  • External force perpendicular to axis
  • Axis rotational moment
  • Bending moment

• Other than the above, loosening not associated with "Reverse rotation motion" such as temperature variations (expansion and contraction), mechanical properties of materials, and wear (see upper rows of [Table 1]).
• Screw loosening fundamental patterns are organized in [Table 1].

[Table 1] Screw loosening fundamental patterns

(2) Typical bolt loosening prevention components

Typical bolt loosening countermeasures and prevention components are summarized in the following table.

[Table 2] Bolt loosening countermeasures and prevention components