Ball couple. Ball screw drives. Ball screws, ball screws. Rated service life in km Lkm

With the advent of industrial production, screw gears began to be widely used in technology, in particular for moving the calipers of metal-cutting machine tools. The development of screw mechanisms was ball screws (ball screws). Their appearance is due to the creation of a new generation of metal-cutting equipment - machine tools with numerical control (CNC).

Functional purpose and device

View of the screw-nut cavity profile: a) arched contour b) radius contour

The purpose of the mechanism under consideration is to convert the rotational movement of the drive into a rectilinear movement of the work object. The transmission consists of two constituent parts: lead screw and nuts.

Screw it is made of high-strength steel grades 8KhF, 8KhFVD, KhVG subjected to induction hardening, or 20Kh3MVF with nitriding. The thread is made in the form of a helical groove of semicircular or triangular section. Depending on the operating conditions of the screw, the cavity profile can have several designs. The most commonly used arched or radius contour.

Enclosing part - screw is a composite node. It has a complex device. Usually it is a housing in which there are two liners with the same grooves as those of the lead screw. Material of insert parts: volume hardened steel grade KhVG, case hardening steels 12KhN3A, 12Kh2N4A, 18KhGT. The inserts are installed in such a way as to provide a preload in the screw-nut system after assembly.

Hardened steel balls made of ShKh15 steel are placed inside the helical grooves, which circulate along a closed path during transmission operation. To do this, inside the body of the nut there are several bypass channels made in the form of tubes connecting the turns of the nut. Their length can be different, that is, the balls can return through one, two turns, or at the end of the nut. The most common is the return to the adjacent coil (DIN system).

Principle of operation

The screw is driven by a drive motor, the nut is fixed motionless on the working body of the machine (caliper, carriage, spindle head, steady rest, and so on). In this case, an axial force arises that acts on the balls placed inside the nut, under the influence of which they begin to roll in closed helical grooves. The reaction force acts on the nut, and since it is rigidly connected to the moving part, it forces the latter to move along the guides of the machine. What is the difference between the operation of a ball screw and a conventional screw drive with a trapezoidal thread, which was previously used on machine tools?

1. When the lead screw of the previous design rotated in the contact zone of two parts, sliding friction arose, characterized by a friction coefficient (bronze on steel, with lubrication) f = 0.07–0.1. In a mechanism with ball elements, rolling friction acts with a coefficient f = 0.0015–0.006. As you can see from the given values, helical ball assemblies require significantly less drive motor power.

2. For precise positioning of the carriage or machine support, before stopping the working body, it is necessary to slow down the speed of its movement. Upon reaching a certain threshold of the minimum speed, microstops - sticking - of the moving unit are possible. At the moment of resumption of motion, its character is determined by the static friction, which, when sliding, significantly exceeds the friction of motion. Because of this, jerks occur that impair positioning accuracy. With rolling friction, this disadvantage is practically reduced to zero.

High-speed or high-speed ball screws

High speed ball screw

An increase in the speed of movement of the nut relative to the screw is achieved by increasing the pitch between the grooves, compared with a standard screw by 3-5 times, for a conventional ball screw with a diameter of 16-32mm, the pitch is 5-10mm, for a high-speed one of the same diameters - 16-32mm and a multiple of screw diameter.

By increasing the speed of movement - losses in rigidity and maximum load on the transmission (to a greater extent) and accuracy (to a lesser extent).

Classification

According to the manufacturing technology, lead screws are:

• rolled- with a helical groove obtained by cold rolling. These screws are produced at a lower cost, so they offer the best price-performance ratio with average manufacturing accuracy (C5, C7, C9).
• sanded- are precision products. After threading and subsequent heat treatment, they are subjected to grinding. Have increased accuracy(C1, C3, C5) and a higher price.

By design:

• Ball screw- made according to the DIN standard. The balls return to the adjacent groove through a reflector groove built into the nut.
• precision- are made by grinding. They can consist of one or two nuts, have a preload (preload) - the elimination of axial clearance in order to improve accuracy during reverses and increase drive rigidity.
• Precision with cage- distinguished by ball return design (no impact) and ground groove profile.
• Precision with rotating nut have a built-in bearing, due to which they have increased movement accuracy.
• Spline shaft with linear bushings flange version. In this case, the shaft performs the function of the inner ring of the bearing. This design is compact and easy to install.
• Console version of the screw. It is used for short lead screws that do not have a second support.

Ball screw specifications

Main settings:
• Screw diameter and pitch - from 16 × 2.5 to 125 × 20 mm.
• The length of the screw rod. Lead screws for CNC machines are usually produced with a maximum length of 2.0–2.5 m, although up to 8 meters are made to order.
• Linear travel speed - up to 110 m/min.
• Transmission accuracy - C1…C10.

Power characteristics for some sizes are given in the table:

Power parameters of ball screws
Diameter × pitch, mm	Load capacity, N		Axial stiffness, N/μm
	static	Dynamic	Housing ball screws	Shellless ball screws
16×2.5	9600	5000	—	230
32×5	37500	17710	700	760
50×10	112500	57750	1000	1100
80×10	197700	66880	1700	1900
125×20	729000	278000	—	2850
Note: axial stiffness is specified for accuracy class C1.

Transmission setting

The choice of ball screws for specific equipment is made in the process of design development, namely at the stage of preliminary design - after the table stroke and the required force on the screw are determined. Then the technical solution is specified:

• Depending on the required degree of accuracy of the drive, choose between conventional and precision gears.
• The design variant of the nut is determined: single, double, the method of returning the balls, the presence of a bearing, and more. A single nut is cheaper, but in case of wear it needs to be replaced, a double nut can be adjusted by grinding the compensator. The ball recirculation system with tubes slightly increases the cost of the nut, but allows the repair of worn channels by replacing the bypass tubes.
• Decide whether or not to support the free end of the screw.
• Specify the nature of the connection of the nut body with the movable unit, as well as the leading end of the lead screw with an electromechanical drive. Perform a dynamic calculation, if necessary, make changes to the design.
• Having completed the assembly of the machine, all components are tested, including the ball screw, according to the test procedure.

Application area

Ball screws are widely used in many industries: machine tools, robotics, assembly lines and transport devices, complex automated systems, woodworking, automotive, medical equipment, nuclear power, space and aviation industries, military equipment, precision measuring instruments and much more. Some examples of using these nodes:

• Feed drives for CNC machines. The first mass-produced in the USSR machining center IR-500 had 3 processing coordinates. Modern systems contain a significantly larger number of linear actuators. For example, Tornos multi-spindle longitudinal turning machines of the MULTI SWISS series have 14 controlled axes.
• Movement of the piston-rack of the steering mechanism of cars (MAZ, KAMAZ, Gazelle).
• Vertical movement of the carriage of the production 3D printer VECTORUS iPro and sPro series.

Manufacturers:

• Steinmeyer (Germany);
• SKF (Sweden);
• MecVel (Italy);
• THK (Japan);
• SBC (Korea);
• HIWIN (Taiwan).

Roller screws (gears, drives) SKF

Roller screw drives - a new stage in the development of drive technology.

The load capacity of the rolling screw-nut gears almost entirely depends on the characteristics of the surfaces at the point of contact between the rolling elements and the screw: diameter, number of contact points, hardness, surface treatment to ensure accuracy and, therefore, uniform distribution of loads between the rolling elements.

In ball screws, the load is transferred from the nut to the screw through balls located in the grooves of the thread. In a ball screw with a single start thread, the ball size is limited to approximately 70% of the thread lead. In this regard, the total contact area is relatively small due to the limited number of full turns of balls in the nut. Show diagram.

In roller screws, the load is transferred through the grooved surface of all cylindrical rollers, which leads to a significant increase in the number of contact points and total area contact with respect to the ball screw. Show diagram.

Roller screw drives are characterized by:

Very high load capacity (static load up to 1500 tons, dynamic load up to 370 tons)
- Very high permissible speed of rotation (for RVP with a diameter of 48 mm - 3300 rpm)
- Very high allowable accelerations (12000 rad/sec sq.)
- Long service life even with continuous operation
- Highest reliability
- Good resistance to aggressive media (dust, sand, ice)
- Good resistance to shock loads and vibrations
- Excellent repeatability of positioning (min. step 0.6 mm)

There are two types of roller screw drives.

(series SR/BR/PR/HR) (show device) withstand the heaviest loads in aggressive environments for thousands of hours, making them suitable for use in applications with very high requirements for load capacity and reliability. A very strong nut is able to withstand shock loads, and the roller synchronization mechanism remains reliable even at high speeds. Large thread pitch and symmetrical nut design allow high speed linear movements.

Planetary roller screw drives are used in broaching machines, presses, machine tools, steel production, tire production, automation of loading and unloading operations, military aviation, tanks, launchers, etc.

(SV/BV/PV series) (show device) allow you to get highest precision positioning through the use of fine pitch threads. The advantages of this design are the minimization of the input torque and the increase in resolution. They are also highly rigid.

Recirculating roller screw drives are used in laboratory and medical equipment, paper production, topographic equipment, telescopes, satellites, etc.

SKF ROLLER SCREW PRODUCTION PROGRAM

SRC Planetary Roller Screw Series:
increase

Cylindrical nuts with axial play
- Thread pitch from 4 to 42 mm

increase

Flanged nuts with end play
- Screw diameters from 8 to 210 mm
- Thread pitch from 4 to 42 mm

increase

BRC - cylindrical nuts with eliminated backlash
- PRU - preloaded cylindrical nuts
- Thread pitch from 2 to 42 mm

increase

BRF - Flanged backlash eliminated nuts
- PRK - flange nuts with preload
- Screw diameters from 8 to 64 mm
- Thread pitch from 4 to 36 mm

HRC - cylindrical nuts with axial play
- HRF, HRP - flange nuts with axial play
- Screw diameters from 60 to 210 mm
- Thread pitch from 15 to 40 mm

ISR - backlash nuts
- IBR - nuts with eliminated backlash
- Screw diameters from 12 to 120 mm
- Thread pitch from 1 to 18 mm

SRR - flange nuts with axial play
- BRR - flange nuts with eliminated backlash
- Screw diameters from 25 to 60 mm
- Thread pitch from 5 to 30 mm

increase

SVC - cylindrical nuts with axial play
- PVU - Cylindrical nuts with preload

- Thread pitch from 0.6 to 5 mm

SVF - flange nuts with end play
- PVK - flange nuts with preload
- Screw diameters from 8 to 125 mm
- Thread pitch from 0.6 to 5 mm

Consider the relationship between the forces acting in a screw pair with a rectangular thread. Let's turn the round of the rectangular thread of the screw along the average diameter d 2 into an inclined plane, and replace the nut with a slider (Fig. 1). The rise of the slider on an inclined plane corresponds to screwing the nut onto the screw.

Rice. 1 - Replace the nut with a slider

As known from theoretical mechanics, interaction strength F between the inclined plane and the slider, which occurs when it moves along the inclined plane, is the resultant of the normal force and the friction force between them and is inclined to the normal n of their contact surface at a friction angle φ.

Let's decompose the force F into two components: axial force F a, acting on the screw pair, and the circumferential force F t rotating the nut when screwing it (in other cases, rotating the screw when screwing it in).

From the force decomposition drawing (Fig. 1) it follows that
where ψ is the angle of the thread.

Obviously the torque T in carving, created by force F t , when screwing in a nut or screwing in a screw,

or

The descent of the slider along an inclined plane (Fig. 2) corresponds to unscrewing the nut or screw. In this case, when expanding the interaction force F between the inclined plane and the slider on the axial force F a and circumferential force F′ t we have

Rice. 2 - Unscrewing the nut

It is obvious that at F′ t ≥0 [which corresponds to the condition tg(φ-ψ)≥0] the thread will be self-braking. Therefore, the condition of self-braking of a rectangular thread is mathematically determined by the condition ψ≤φ. When lifting the slider on an inclined plane, the driving force F t (Fig. 1) to a height equal to the thread stroke P h , work of driving forces

and the work of the forces of useful resistance

Efficiency η of a screw pair with a rectangular thread when screwing in a nut or screwing in a screw.


or

From the analysis of the formula it follows that for a self-braking screw pair, where ψ Let's consider the force relations, self-braking conditions and efficiency, a screw pair with a triangular or trapezoidal thread. Since the reasoning and conclusions for these threads are the same, we will consider them in relation to a triangular thread. If in the considered screw pair we replace the rectangular thread with a triangular one, then the friction force in the thread, and hence the circumferential force of the screw pair, will have different values. Let us determine the friction forces and establish the relationship between the friction forces in rectangular and triangular threads. To simplify the conclusions, the angle of inclination of the thread will be taken equal to zero. Friction force for rectangular thread (Fig. 3)

where ƒ is the coefficient of friction. Friction force for triangular (fig. 4) or trapezoidal threads

where α is the angle of the thread profile,
ƒ′ - reduced coefficient of friction:

Rice. 3 - Friction force for rectangular thread

It follows from the formula that, compared with a rectangular thread, triangular and trapezoidal threads have more friction. For normal metric threadα=60° and ƒ′=1.15ƒ, for a trapezoidal thread α=30° and ƒ′=1.04ƒ, therefore, friction in this thread is greater than in a rectangular thread, but less than in a triangular one.

Rice. 4 - Trapezoidal thread friction force

Obviously, the ratio between the friction coefficients ƒ and ƒ′ corresponds to the ratio between the friction angles φ and φ′ where φ′ is the reduced friction angle:

The ratios between forces in rectangular and triangular threads are similar. Therefore, by analogy with the formulas, it follows that for a triangular or trapezoidal thread, the circumferential force
torque in the thread
the self-braking condition is determined by the expression ψ≤φ′, the efficiency
and for a self-braking screw pair, where ψ Rice. 5 - End bearing surface of the nut

The friction torque T f at the end face of the nut or screw head when they are screwed in is determined as follows. The end bearing surface of the nut or screw head (Fig. 5) is taken to be annular with an outer diameter D, equal to the opening of the key, and an inner diameter d 0 equal to the diameter of the hole for the bolt, screw or stud. It is generally accepted that the pressure on the supporting surface is distributed evenly, i.e.

Thus, the frictional moment at the end of the nut or screw head

or finally

To simplify calculations, it is often assumed that the resultant frictional force ƒF on the bearing surface of the nut or screw head acts tangentially to a circle of average diameter d c , the bearing surface and the moment

where

The last formula in technical calculations gives quite sufficient accuracy.

It is obvious that the torque of screwing in the nut or screwing in the set screw

To create machine tools with program numerical control, it is necessary to use ball screws. They differ not only appearance but also by design. To select a specific model, you should familiarize yourself with the structure and components of the ball screw in advance.

Purpose of ball screws

All types of ball screws for CNC machines are designed to convert rotational motion into translational. Structurally, they consist of a body and a lead screw. They differ from each other in size and technical characteristics.

The main requirement is to minimize friction during operation. To do this, the surface of the components undergoes a thorough grinding process. As a result, during the movement of the lead screw, there are no sharp jumps in its position relative to the housing with bearings.

Additionally, to achieve a smooth ride, not sliding friction relative to the pin and body is used, but rolling. To obtain this effect, the principle of ball bearings is applied. Such a scheme increases the overload characteristics of ball screws for CNC machines, and significantly increases efficiency.

Main Components of a Ball Screw:

• lead screw. Designed to convert rotational motion into translational. A thread is formed on its surface, the main characteristic is its pitch;
• frame. During the movement of the lead screw, displacement occurs. Various components of the machine can be installed on the body: cutters, drills, etc.;
• balls and inserts. Necessary for smooth running of the body relative to the axis of the lead screw.

Despite all the advantages of this design, ball screws for CNC are used only for medium and small machines. This is due to the possibility of screw deflection when the housing is located in its middle part. Currently the maximum allowable length is 1.5 m.

The screw-nut transmission has similar properties. However, this scheme is characterized by rapid wear of components due to their constant friction with each other.

Ball screw applications

The relative simplicity of design and the possibility of manufacturing a ball screw with different characteristics expands its scope. Nowadays, ball screws are an integral part of homemade CNC milling machines. Well, the scope is not limited to this.

Due to their versatility, ball screws can be installed not only in CNC machines. Smooth running and practically zero friction make them indispensable components in precision measuring instruments, medical devices, and mechanical engineering. Often for picking homemade equipment take spare parts from these devices.

This is made possible by the following properties:

• minimization of friction losses;
• high coefficient of load capacity with small dimensions of the structure;
• low inertia. The movement of the body occurs simultaneously with the rotation of the screw;
• No noise and smooth running.

However, the disadvantages of ball screws for CNC equipment should also be taken into account. First of all, they include a complex hull design. Even if one of the components is slightly damaged, the ball screw will not be able to perform its functions. There are also restrictions on the speed of rotation of the screw. Exceeding this setting may cause vibration.

To reduce the axial clearance, the assembly is carried out with an interference fit. To do this, balls of increased diameter or two nuts with axial displacement can be installed.

Characteristics of ball screws for CNC equipment

To select the optimal model of ball screw for machine tools with numerical control, you should familiarize yourself with the technical specifications. In the future, they will affect the performance of the equipment and the time of its non-repair operation.

The main parameter of a ball screw for CNC machines is the accuracy class. It determines the degree of error in the position of the mobile system according to the calculated characteristics. The accuracy class can be from C0 to C10. The displacement error must be given by the manufacturer, indicated in the technical data sheet of the product.

Accuracy class	C0	C1	C2	C3	C5	C7	C10
Accuracy at 300 µm	3,5	5	7	8	18	50	120
Error per screw turn	2,5	4	5	6	8

In addition, when choosing, you need to consider the following parameters:

• the ratio of the maximum and required motor speed;
• total length of lead screw thread;
• average load indicators for the entire structure;
• value of axial load - preload;
• geometric dimensions - the diameter of the screw and nut;
• motor parameters - torque, power and other characteristics.

These data must be pre-calculated. It should be remembered that the actual characteristics of ball screws for CNC equipment cannot differ from the calculated ones. Otherwise, it will cause the machine to work incorrectly.

The number of revolutions of the balls in one circle will determine the degree of torque transfer from the shaft to the body. This parameter depends on the diameter of the balls, their number and the cross section of the shaft.

Installing a ball screw on a CNC machine

After choosing the optimal model, it is necessary to think over the installation scheme of the ball screw on the CNC machine. To do this, a design drawing is preliminarily drawn up, other components are purchased or manufactured.

During the execution of the work, it is necessary to take into account not only specifications ball screw. Its main purpose is the movement of machine elements along a certain axis. Therefore, you should think in advance about the fastening of the processing unit to the body of the ball screw for CNC machines. It is necessary to check the dimensions of the mounting holes, their location on the body. It should be remembered that any mechanical processing of the ball screw may lead to negative changes in its characteristics.

The order of installation in the body of the CNC machine.

1. Determination of optimal technical characteristics.
2. Shaft length measurement.
3. Creation of a scheme for pairing the mounting part of the shaft with the motor rotor.
4. Installing the gear on the machine body.
5. Checking node health.
6. Connection of all main components.

After that, you can perform the first test run of the equipment. In the course of work there should be no oscillations and vibrations. If they appear, perform an additional calibration of the components.

If the ball screw breaks down during the operation of the CNC machine, the gear repair can be done independently. To do this, you can order a special kit. You can get acquainted with the features of the restoration work in the video: