Monday, November 30, 2015
Ballscrew Design & Lifetime Criteria Explained
Pre-Loading of Ballscrews One of the distinctive features of a ball screw is the ability to have increase rigidity by preloading. In order to increase the rigidity of a ball screw, "preloading" must be applied to reduce an axial clearance between a ball nut and the screw shaft. Precise positioning can be achieved by the application of preloading. However, it is of great importance to apply only the proper amount of preloading in order to prevent excessive heat-generation and premature termination of ball screw's expected life. There are various methods of ball screw preloading as shown in the following. Kuroda uses: Oversized Ball Preload for light to medium loading applications, and Double Nut (Pin Method) for medium and higher loading applications. What is dynamic load rating? Basic Dynamic Load Rating is a bearing term that represents an applicable constant load (in direction and magnitude) where a bearing will achieve 10 6 (1,000,000) rotations of Rated Life. The Rated Life of a bearing is defined as: Total number of rotations reached by 90% of a specimen group of bearings, without sustaining any material damages due to rolling stress. This in a Ball Screw perspective, will mean: An axial load applied to a group of same ball screws where 90% of them will reach 1,000,000 rotations without suffering any Flaking. To put all the above in a more practical terms: For instance, our catalog shows that a ball screw, 10mm shaft diameter with a single ball nut with 1-ball-row x 1.5 turns, has a Dynamic Load Rating of 440daN. This means that when we apply 440daN of axial load on 100 specimens of this type, 90 specimens or more will reach 1,000,000 rotations without any damages. (One million rotations will equate to 500 hours if the ball screw is operated at 33.3 rpm) In other words, the Basic Dynamic Load Rating provides a basis of Life Calculations based on the size and type of ball screws. This rating will be the same for different ball nut types such as Single Nut, Integral Nut and Double Nut, as long as the shaft diameter, lead, number of recirculating circuits, and ball diameter are the same. What is basic static load rating? Basic Static Load Rating is a bearing term explained as: a static (non-operating) load that causes a permanent deformation at a contacting point of the highest stress, on both the rolling element and the bearing race, in the summed amount of 1/10,000th of the ball diameter. The above principle also applies to ball screws as: an axially applied static load that causes a permanent deformation on bearing balls and screw groove as a sum, in the amount of 1/10,000th of the ball diameter. Generally, the Basic Static Load Rating does not become much of an issue in ball screw usage. The Basic Dynamic Load Rating is used for Life Calculations, but in case where the ball screws are used at low rotational speeds, especially below 10 rpm, it is necessary to select ball screws with Basic Static Load Rating 1~3 times that of the actually applied maximum axial loads. The Basic Static Load Rating will be the same for different nut types such as Single Nut, Integral Nut and Double Nut, as long as the shaft diameter, lead, number of recirculating circuits, and ball diameter are the same. How is Ballscrew life calculated? The Life of a ball screw is defined in a unit as the total number of rotations reached before surface-flaking of the balls or the ball groove starts to appear, due to repeated stress on their materials, and can be calculated with the Basic Dynamic Load Rating (C). It is not advisable to use the Basic Dynamic Load rating as a basis for a life calculation where the application requires the ball screw to operate at less than 10 rpm, due to large errors in predicted results. For this reason, use the Basic Static Load Rating (C0) when calculating for low rotational speed applications. The Basic Static Load Rating is: an axially applied static load that causes a permanent deformation on steel balls and screw groove as a sum, in the amount of 1/10,000th of the ball diameter. The life of a ball screw can be calculated. Over-designing the life parameter will result in unnecessarily high costs and excessive size of the mechanism. What is backlash? The term "Backlash" originated from gear engineering discipline. In JIS terminology, this word is described as: Play that exists between a pair of engaged gears' tooth surfaces. When described as a distance of an arc on the engagement pitch circle, it is called "Circumference Direction Backlash", when described as the shortest distance between two opposing tooth surfaces is called "Normal Line Backlash". What is critical speed? An elongated cylindrical object such as a ball screw shaft will always have a certain amount of sag at the middle due to its own mass (weight). If such cylindrical object (a screw shaft) is held by rotary bearings at its ends and rotated, the sag would increase as the rotational speed is increased and may eventually reach a speed point that is destructive to the shaft. This potentially destructive rotational speed is called the "Critical Speed". It is also referred to as "Whirling Speed" or "Whipping Speed". The Critical Speed parameter should be paid a high degree of attention when designing ball screw driven systems. The Critical Speed can be obtained by calculating, or from "Allowable Speeds" nomogram provided in our catalog. It can be observed from the nomogram that smaller the diameter and longer it is, the shaft will become critical at lower speeds (Critical Speed is low). In addition, any imbalances in other rotating elements within the system will contribute in abnormal vibrations. Careful design considerations ensuringto eliminate any axial concentricity errors in the support bearings, and to avoid any bending forces on the screw shaft. Since the nuts travel in ball screws acting as moving supports, and the operation of ball screws involve reversing at their stroke ends, it is generally understood that the oscillation amplitudes of ball screws are smaller than that of common drive shafts.
When designing with ball screws, evaluation of "DmN value" is also required in addition to the Critical Speeds.
If ball screws are operated beyond their critical speeds, the accuracy of the machinery systems may be degraded due to the vibrations generated by the screw shaft. It is recommended that all ball screws to be operated at 80% or below of their Critical Speeds. Furthermore, the Critical Speeds may be improved by evaluating the shaft mounting methods (Fixed - Supported, etc.). Selections in shaft-end bearings may also help to extend the high-speed capabilities.
As a general rule of thumb, please contact Kuroda Technical Support for consultations if the ball screws are to be operated at speeds higher than 2,000 rpm.
What is the purpose of lubrication?
To "lubricate" means to reduce the frictions of contacting surfaces in machinery components. The purpose of Lubrication in ball screw applications is to reduce friction and wear, provide anti-corrosion, cooling, and foreign substance displacement measures. All these measures are to maintain ball screw's excellent characteristics and performances.
Lubricants are divided in two major categories, Greases and Oils, and must be selected based on application specific requirements. Grease is often the general preference as the lubricating medium for ball screws due to lower cost and ease of application. The oil lubrication requires frequent application with centralized lubrication systems, and is rarely used other than in machine tool and other special applications where constant removal of foreign matter and heat are required.
Kuroda use Albania Grease S2 for our standard without any suggestion from the customers. Use of Lithium soap-based grease is recommended for ball screws because it has excellent watertight and heat-resisting characteristics, and use of spindle oil or turbine oil (ISO VG32-68) is also recommended as lubricant.
I want to know about accuracy grades
The Ball Screw Accuracy Grade scale is divided into eight classes: Precision Grades - C0, C1, C2, C3, C4, C5, and the General Grades: C7 and C10. Higher the number, lower the accuracy grade.
The accuracy of a ball screw is defined by three major parameters, namely: Lead accuracy, Mounting interface accuracy, and Preload-torque variation in percentage. The JIS standards set forth the allowable values for each accuracy grade.
(Example) A ball screw with C5 accuracy grade has: Lead accuracy of C5, Mounting interface accuracy of C5, and Preload torque variation percentage of C5
Ball screw accuracy grade should be chosen to satisfy the positioning accuracy requirement of the applicable machinery system. Refer to our catalog data. Kuroda recommends the following guideline, based on our application experiences.
I want to know about ballscrew noise
Due to the higher speed requirements and changes in environmental concerns, demands for lowered ball screw noise are on an increase. Quieter ball screws are needed especially in the areas of Factory Automation and Office Automation.
There are three possible sources of ball screw noises: from the ball screw itself, errors in mounting accuracy, and from the entire machinery as a whole.
Ball screw itself
Since the ball screws contain rolling elements that make contacts with other components, recirculating noises and rolling noises are unavoidable. Based on the researches on this subject, it is now known that small waviness on the screw shaft's groove surfaces have large effects on the ball screw noise.
Errors in mounting accuracy
Mounting inaccuracies in parallelism and perpendicularity during the installation can create uneven loads on the ball screw system, causing abnormal noises. This can be examined by temporarily loosening the mounting bolts and test-run the system to look for any reductions in the noise.
Machine as a whole
Some resonance may occur within the machine as a whole, including the ball screw. The possible causes may be the drive motors, feedback systems, and inadequate machine rigidity.
What about material and heat treatment
The hardness of ball groove on the shaft largely influences life expectancy of a ball screw. In addition, the shaft must possess a strength required as a drive shaft.
The following table shows the material composition and heat treating details of a ball screw system.
Ball screw shafts and nuts are surface hardened for the following reasons.
(1) Ball races are hardened since it largely affects life.
(2) They are not through-hardened to avoid becoming brittle and prone to breakage.
n other cases where high temperature immunity and anti-corrosive property are desired, stainless steel (SUS440C) is used. The stainless steel components are heat treated to achieve HRc56~59 hardness.