The Fundamentals of Ball Screw
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The Fundamentals of Ball Screw
 
Article Introduction
The function of ball screws is based on the principle of converting linear motion to rotary and vice versa by a train of re-circulating balls interposed between hardened ball tracks in ball nut and shaft.

Article Description
The best way to win production targets is to incorporate faster functions. It is here that the help of advanced technology is most needed to support varied needs. An example is the development of ball screw in replacement to ACME, a brilliant upgradation of technology for demanding applications. As the transmission of the axial force is by rolling contact and not sliding friction, the reduction in friction gives low wear rates, high efficiency and maximum repeatability of operation.

The precision of a ball track in semi-circular and gothic forms is in close conformity to high-level accuracy to achieve high system stiffness, response and repeatability of accurate positioning. Semi-circular Forms: Incorporating close conformity ball-tracks (51-1/2%) with acceptability of limited backlash achieving higher load capacity.

Gothic Forms:
These forms are semi- circular arcs produced with displaced centres in order to control backlash (generally with use in single ball nuts). Has increased conformity (54%) in order to reduce ball scuffing.

Ball screws are divided basically in two groups, as single nut and double nut assembly, for varied applications.
Single Nut Assembly is for loads in one direction, with limited acceptability in axial backlash. Double Nut Assembly is for preloads to achieve specific rigidity performance with total elimination of backlash.

Features
High Efficiency: As the transmission of axial force is by rolling contact and not by sliding friction. Marc ball screws have efficiency greater than 90%.
Smooth Operation:
Substitution of rolling contact for the sliding metal-to-metal contact minimises starting friction eliminates the stick slip tendency where a slow, smooth linear motion is required.
Wear& Life:
The balls rolling in the hardened grooves reduce wear to minimum and thus increase the life of the ball screws.
Accuracy:
Marc ball screws are ground, assembled and inspected with efficient and sophisticated facilities at a temperature control.
Back Lash:
Marc ball screws can eliminate back lash (axial clearance) by preloading two nuts, thus increasing in stiffness.
Groove Profile:
Supplied with Gothic arch grooves.
Reliability:
Marc precision ball screws are the result of indigenous rational design, selected raw materials, heat treatment techniques and advanced process techniques.

Re-Circulation Systems
Marc precision ball screws have two kinds of re-circulation systems: one is called External re-circulation type, in which the balls are re-circulated in the ball return tube set up on the nut. Again, the external system is classified as:
(i) Tubes inside the nut locating diameter, and
(ii) Tubes outside the nut-locating diameter.
Another design is called internal re-circulation type, in which the balls make only one revolution of the screw shaft and the circuit is closed by a ball return cap/deflector in the nut allowing the balls to cross over adjacent ball tracks. In this the deflectors are located inside the body nut.

Factors for Selecting Ball Screw Assemblies
The following factors are to be considered in the selection of ball screw assembly required for a given application:
·Load conditions
·Critical speed
·Buckling load
·Degree of accuracy required
·Service life
·Stiffness
·Nut specification
·Mounting method
.Screw end configuration, and
·Lubrication

Lubrication
Ball screws require appropriate lubrication for effective functioning, either by greasing or oiling. Anti-friction bearing oil is recommended for ball screws. Lithium soap based grease is recommended for ball screw greasing. The basic oil viscosity requirement depends on the speed, working temperature and load condition of the application. When the working speed is higher and the working load is lower, low viscosity oil is better. When the working speed is lower and the working load is heavy, high viscosity oil is preferred.

Generally speaking, oil with viscosity of 32-68 CST at 40 (ISO. VG 32-68) is recommended for high-speed lubrication (DIN 51519) and viscosity above 90 CST at 40(ISO, VG 90) is recommended for low speed lubrication. In high speed and heavy load applications, the use of a forced coolant is necessary to lessen the temperature.
Posted : 8/12/2005

 
 
The Fundamentals of Ball Screw