Ball Screw

Ball screws are feed screws that provide high efficiency due to the rolling motion of the balls between the screw shaft and nut.
The drive torque is less than one-third that of a conventional sliding screw, making it ideal for power saving of the drive motor.

Handling Precautions

Types

In the DIN standard compliant Ball Screw, balls under a load roll in the raceway cut between the screw shaft and the nut while receiving the axial load, travel along the groove of a deflector embedded inside the nut to the adjacent raceway, and then circulate back to the loaded area. Thus, the balls perform infinite rolling motion.

The ball cage enables this ball screw to have low noise, low torque fluctuation, and long-term maintenance-free operation.

This ball screw uses a screw shaft that is ground with high precision.

This type of ball screw is mass produced by cutting standardized screw shafts of precision ball screws to fixed lengths. The shaft ends can easily be modified with additional machining.

These ball screw products are fabricated with standardized shaft end shapes. The Model SDA-VZ, which features a compact, ISO-compliant nut, has been added to the lineup.
These products can be used in combination with dedicated support units to make assembly design easy and significantly faster.

The Model BNK is a ball screw with finished shaft ends machined to match the support unit.

These ball screws use screw shafts rolled with high precision.

The use of a guide plate system provides a compact design with a round outer diameter for the nut. The screw shaft is roll-formed with a high degree of precision to ensure smooth operation.

These rotary-nut ball screws have an integrated structure consisting of a ball screw nut and a support bearing.

The rotary ball screw is a rotary-nut ball screw with a structure integrating a ball screw nut and a support bearing.

The ball screw grooves and the ball spline groove cross one another on a single shaft and the nuts of the ball screw and the ball spline have dedicated support bearings directly embedded on the circumferences of the nuts.

The Model DSP transmits a large thrust with smooth motion due to its structure which integrates a linear guide ball spline and a ball screw as the drive source.

Support units are dedicated bearing components used as a set with ball screws.
They come in several types: models EK, FK, FKT, EF, and FF, which are tailored to fit Model BNK and Model SDA-VZ precision ball screws with finished shaft ends; models EK-L and EF-L, which are for low shaft centers; and models BK and BF, which are standardized for general ball screws.
The support unit on the fixed side includes a JIS Class 5-compliant angular bearing provided with an adjusted preload. The support unit on the supported side uses a deep-groove ball bearing.

The nut bracket is standardized for the standard ball screw assembly provided with finished shaft ends. It is designed to be secured directly on the table with bolts. Since the height is low, it can be mounted on the table only using bolts.

The lock nut for ball screws is capable of fastening the screw shaft and the bearing with a high accuracy. The provided hexagonal socket-head setscrew and the set piece prevent the lock nut from loosening and ensure firm fastening.

Features

Driving Torque One Third that of a Sliding Screw

In a ball screw, balls roll between the screw shaft and the nut to achieve high efficiency. The required driving torque is only one third of the conventional sliding screw (see Fig. 1 and Fig. 2). As a result, it is capable of not only converting rotational motion to linear motion, but also converting linear motion to rotational motion.

Calculating the Lead Angle

β	: Lead angle	(°)
d_P	: Ball center-to-center diameter	(mm)
Ph	: Feed screw lead	(mm)

Relationship between Thrust and Torque

The torque or thrust generated when thrust or torque is applied is obtained from equations (1) to (3).

● Driving Torque Required to Gain Thrust

T	: Driving torque	(N･mm)
Fa	: Frictional resistance on the guide surface	(N)
Fa = μ × mg
μ	: Frictional coefficient of the guide surface
g	: Gravitational acceleration	(9.8 m/s²)
m	: Mass of the transferred object	(kg)
Ph	: Feed screw lead	(mm)
η1	: Positive efficiency of feed screw	(Driving Torque One-Third That of a Sliding Screw see Fig. 1)

● Thrust Generated When Torque is Applied

Fa	: Thrust generated	(N)
T	: Driving torque	(N･mm)
Ph	: Feed screw lead	(mm)
η1	: Positive efficiency of feed screw	(Driving Torque One-Third That of a Sliding Screw see Fig. 1)

● Torque Generated When Thrust is Applied

T	: Torque generated	(N･mm)
Fa	: Thrust generated	(N)
Ph	: Feed screw lead	(mm)
η2	: Reverse efficiency of feed screw	(Driving Torque One-Third That of a Sliding Screw see Fig. 2)

Example Driving Torque Calculation

When moving an object with a mass of 500 kg using a screw with an effective diameter of 33 mm and a lead length of 10 mm (lead angle: 5°30'), the required torque is obtained as follows.

Rolling guide (μ = 0.003)
Ball screw (from μ = 0.003, η = 0.96)

Frictional resistance on the guide surface
Fa = 0.003 × 500 × 9.8 = 14.7N

Driving torque

Rolling guide (μ = 0.003)
Ball screw (from μ = 0.2, η = 0.32)

Frictional resistance on the guide surface
Fa = 0.003 × 500 × 9.8 = 14.7 N

Driving torque

Ensuring High Accuracy

Ball screws are ground with the highest level of equipment at a factory with strict temperature controls. Their accuracy is assured under a thorough quality control system that covers assembly to inspection.

Conditions
Model No.: BIF3205-10RRG0+903LC2

Table 1: Lead Accuracy Measurement

Unit: mm

Item	Standard value	Actual measurement
Directional target point	0	-
Representative travel distance error	±0.011	-0.0012
Fluctuation	0.008	0.0017

Capable of Micro Feeding

The ball screw requires a minimal starting torque due to its rolling motion, and does not cause a slip, which is inevitable with a sliding motion. Therefore, it is capable of accurate micro feeding. Fig. 4 shows a travel distance of the ball screw in one-pulse, 0.1-m feeding. (LM Guide is used for the guide surface.)

High Rigidity without Backlash

Since a ball screw is capable of receiving a preload, it can be preloaded with an axial clearance below zero, allowing it to achieve high rigidity. In Fig. 5, when an axial load is applied in the positive (+) direction, the table is displaced in the same (+) direction. When an axial load is provided in the reverse (-) direction, the table is displaced in the same (-) direction. Fig. 6 shows the relationship between the axial load and the axial displacement. As indicated in Fig. 6, as the direction of the axial load changes, the axial clearance occurs as a displacement. Additionally, when the ball screw has a preload applied, it gains a higher rigidity and a smaller axial displacement than a zero clearance in the axial direction.

Capable of Fast Feed

Since the ball screw is highly efficient and generates little heat, it is capable of a fast feed.

High Speed Example

Fig. 7 shows a speed diagram for a large lead rolled ball screw operating at 2 m/s.

Conditions

Item	Description
Sample	Large Lead Rolled Ball Screw WTF3060 (Shaft diameter: 30 mm; lead: 60 mm)
Maximum speed	2 m/s (Ball screw rotational speed: 2,000 min^-1)
Guide surface	LM Guide Model SR25W

Item

Description

Sample

Large Lead Rolled Ball Screw

WTF3060

(Shaft diameter: 30 mm; lead: 60 mm)

Maximum speed

2 m/s

(Ball screw rotational speed: 2,000 min^-1)

Guide surface

LM Guide Model SR25W