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.
Therefore, it is important to properly size the ball screw based on its life calculations. The following is the typically accepted guideline. Machine Tool 20000 hrs. Industrial Automation 10000 hrs. Equipment Automation Control 15000 hrs. Measuring instruments 15000 hrs. The following Surface-Flaking progression of ball screws beyond its life expectancy has been observed as the result of Fatigue Life Tests. Some round "pitting" 0.2mm in diameter appear on the surface of the ball nut groove initially, followed by appearance and propagation of "cracks", and finally resulting in "flaking" of the entire surface of the ball screw shaft. 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".
Generally, some play is required in machinery systems. Gears will not function if all back is eliminated. It is inevitable to have small back-play in any gear train in operation. Same can be said for Triangular and Trapezoidal Screw systems, and it would be difficult to hand-rotate without some "play". However, it is possible to eliminate this backlash in ball screw systems, making possible to ensure accurate positioning in reversing applications. Causes for ball screw backlash are: 1) Axial clearance 2) Axial elastic deformation of nut 3) Elongation and contraction of screw shaft 4) Axial deformation due to tortional forces The entire drive train mechanism must be taken in consideration when considering the subject of Backlash. There is a method of backlash reduction caused by axial clearance and deformation of the contacting surfaces. It is called "Preloading" and is applied during ball screw assembly. This "Preload" is not to be confused with applied load on the system, but should be understood as internally built-in load. 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.
For more details on how Kuroda Jena Tec can supply your requirements please visit our websites on

Kuroda Jena Tec Appoints Rolman World as MEA MRO Distributor

Kuroda Jena Tec and Rolman World are pleased to announce the Agreement for the Distribution of Kuroda Jena Tec products across the Middle East & Africa (MEA). Rolman World is renowned for its value-added brands, and for its technical services in the region. Considered MEA’s largest privately owned Authorised Distributor of bearings, belts, chain, seals and related products and services, Rolman World has over 300 employees, and provides services via its UAE Headquarters, 16 regional subsidiaries, technical centres and extensive distribution network. This Partnership Agreement allows Rolman World to extend the capability of their goods and services to include ballscrew repair and replacement to their large, existing – and fast-growing – End User customer base.
Tim Green, Senior VP of Global Business Development commented “ In addition to our existing high quality product range of bearings and linear suppliers, including THK, NSK and IKO, we are extremely pleased to enter into a partnership with the Kuroda Jena Tec Group for the development of our ballscrew MRO services in the region. Kuroda Jena Tec’s high quality Japanese and German linear product lines, combined with their specific MRO expertise and short delivery, complements our core competence in MRO, and ensures that we can more fully support our MEA customers’ requirements”. Paul Ward, CEO of Kuroda Jena-Tec Holdings commented “We’re really pleased to make an Agreement with such a reputable company as Rolman World to represent our products and services in the Middle East & Africa. Our specific capability in Maintenance, Repair & Overhaul of linear products such as ballscrews and linear actuators means we can actively grow this capability together with Rolman World. Our combination of high quality OEM ground, whirled and rolled ballscrews in stock, machined to print or sample on a short delivery offers the MRO customers the highest level of service in the region today”. For more information on either company, please visit or

Monday, October 26, 2015

Kuroda Jena Tec : Principles of Ballscrew Repair

Kuroda Jena Tec's ball screw repair technicians can recondition or repair ballscrews of any make or configuration with a range of transfer systems. At our specialised repair and testing facilities, we can recondition ballscrews up to 15m in length and 200mm in diameter. Additionally, we can replace ballscrews, offering a re-engineered matched replacement utilizing our existing inventory, up to 80mm in diameter.
Every repair evaluation includes a health check and free of charge assessment report. Our skilled technicians will disassemble, clean, inspect and determine cause of failure. The ballscrew is initially inspected for shaft straightness, bearing journal total indicated runout (TIR), fine thread and journal condition and wear to the shaft.
The ballscrew is then stripped and all parts are vapour de-greased and checked internally for Ball transfer system damage, Brinelling, fatigue, corrosion and damage to raceways. Each individual ballscrew shaft raceway is then polished, fine threads are cleaned and dressed out if necessary. The ballscrew is reassembled using new, high quality chrome steel balls to the correct pre-load specification. Lastly, the ballscrew is put through a series of tests and one final inspection to ensure precision and quality. If it is predetermined that the ballscrew is beyond economical repair, Kuroda Jena Tec offers reverse engineering and will quote for a replacement ballscrew manufactured in accordance with the sample. This should also be considered if the ballscrew is in a poor but running condition. Ballscrew sizes in diameter up to 80mm and in precision grade are held in stock in the UK and USA which can be retrofitted for replacements. Sizes over this diameter are supplied through Kuroda Jena Tec's German facility supported by the regional technical office. Warranties offered on repair services are dependent on repair assessment, ranging from zero warranty (where a replacement is recommended) to 6 months maximum. For more details on how Kuroda Jena Tec can supply your requirements please visit our websites on

Kuroda Jena Tec : We Make Other Stuff Too!

Kuroda Jena Tec is increasingly known as a manufacturer and supplier of linear products for high precision ballscrews. However what is less widely known is the full range of high quality components for the machine tool and automation manufactured by the Kuroda Jena Tec group globally summarised below.
Motorised and belt driven spindles are designed and manufactured in our UK facility and provided globally to high calibre machine tool customers for grinding, milling, turning applications. An experienced and established design team works with OEM customers to meet the exact customer design specification. Utilising precision grade bearings spindles can be offered in a variety of round body or prismatic housings up to 100,000rpm with numerous additional technical features including ATC, Through spindle coolant, HSK, ISO, BT tool interfaces. Design and manufacture of multi head spindles for transfer lines remains a core competence of the UK team as well modular grinding heads for use on VTL’s or turning machines to grind complex components. Specifically related to the UK, Kuroda Jena Tec is the exclusive distributor of GMN spindles in the UK and Ireland and provides sales and service support for this popular and high quality German range.
Precision Gauges - Since its founding in 1925, Kuroda Precision Industries Ltd. has continued to advance with the history of gauges in our country, and has been striving for improvement in the product quality and reliability. As a result, Kuroda Jena Tec has successfully received the highest class evaluation as a pioneer of gauge manufacturer, such as an authority to indicate JIS "Grade 1" thread gauge mark for the first time in Japan and a production license for API gauges first time ever in the East. The importance of gauges has been recognized anew, because they play a significant role in quality control and quality assurance and the traceability of measurement has become a necessary condition in the quality management system represented by ISO 9000 Series in recent years.
As an authorized company for length section calibration, Kuroda Jena Tec believes firmly that it will be of service to the establishment of traceability of customer's measurement equipment to the national standard through the calibration and certification of gauge blocks and other various gauges as well as measurement equipment. In addition, our ultra-precision machining technology accumulated through the development of gauges is the essential technology contributed to the expansion of all of KURODA's products, and this technology is efficiently employed in tooling, precision grinding ball screws, pulse encoders, CNC surface grinding machines, super polishing machines, surface profile measuring systems, precision press dies, etc. Spindle Test Bars - Kuroda Jena Tec’s range of precision gauging also allows our Japanese production facility the capability to produce high quality spindle test bars in HSK, ISO or BT taper interface.
MQL Systems - Kuroda Jena Tec manufacture MQL systems in Japan and supply to leading global machine tool manufacturers. MQL as an acronym (Minimum Quantity Lubrication) is becoming slowly more understood in the machine tool industry as its clear technical and cost benefits start to be delivered for users looking to reduce consumption costs and improve technical and environmental factors for their machining processes. Previously MQL had limited applications and was frequently associated with specialist processes such as turning or specialist mould and die finishing with ball end-mill applications. MQL utilises a soluble and/or bio-degradable cutting oil delivered in a fine mist to the machining area via either externally delivered nozzles or via integrated tooling to the work piece. It is used as an alternative to either dry machining where tool life is reduced or in place of high pressure flood coolant. Firstly, the disadvantages of flood coolant are numerous such as the high running cost of coolant pumps, maintenance of coolant systems, cleaning of the work area of the machine tool and particularly in summer or warm conditions the unpleasant odour sometimes emitted by poorly maintained coolant systems. It is probably a common viewpoint in the machining industry that a frequent observation of machine shops using flood coolant require a high level of maintenance not only in monitoring coolant condition but also clearing up spills from leaks and so on that present a health and safety hazard in the work place.
While therefore it would be ideal to implement dry machining the effect on tool life, heat management and process performance means that this is not possible in most cases. Where dry machining is successfully employed, the process performance can be inhibited by the need to balance productivity against surface finish and tool life and maintenance costs. Therefore even where users have a defined dry cutting process, MQL can potentially assist with extending tool life and increase machining performance in terms of surface finish, cutting forces and process capability. Inverter Drives - As a spindle manufacturer, Kuroda Jena Tec is also frequently asked to package spindle inverter drives and supply a package of components to the customer. With in-house electrical design capability, Kuroda Jena Tec UK provides a complete design, supply and commissioning service for inverter drives systems related to precision motorised spindles. This is particularly useful when considering a modular grinding attachment or retrofit and modernisation of a machine tool. Kuroda Jena Tec can therefore provide a complete solution for your retrofit installation.
Hydraulic Work Holding - One of the major challenges in machining operations is the ability to clamp and hold workpieces reliably and repeatedly. Kuroda Jena Tec in Japan have over 20 years design and production experience providing hydraulic clamping utilising thin walled expanding shells to clamp components with 1 micron repeatability. Typically used in grinding operations the constant hydraulic pressure reduces workpiece distortion avoiding twisting or buckling once the workpiece is released.

Thursday, October 1, 2015

Launch of “Fastmake” DIN Ballscrew Machining Service

Kuroda Jena Tec has launched its “Fastmake” service in Europe and North America to service the needs of the market for high precision ballscrews on short delivery. Kuroda Jena Tec now stocks in Michigan and UK from diameter 16mm to diameter 80mm in various quality grades in popular pitch configurations including precision ground, whirled and rolled and is able to provide standard delivery times of 3 weeks including end machining. For express shipments, delivery can be made within 1 week or for emergency breakdown purposes in one day in some cases. Predominantly in DIN sizes, Kuroda Jena Tec also stocks JTs3 series ballnuts with a slightly softer mounting flange without holes which allows the company to customise the hole pattern quickly and easily to suit non-DIN hole mounting. Further flexibility is offered via JTs7 style ballnuts offering plain threaded end fixing without flange offering even more flexibility in mounting options. End machining customisation includes the facility for hard turning of the hardened stock shaft with straightening and journal grinding capability which offers the customer guaranteed OEM quality whether servicing the requests of OEM or MRO customers. In addition to “Fastmake” Kuroda Jena Tec can offer a wide variety of other sizes and pitches made to order from diameter 5mm up to 125mm with standard or customised ballnuts from their Japanese and German production facilities. For more information please contact your local sales office or authorised distributor via

Kuroda Jena Tec Harmonisation Creates Global Ballscrew Brand

Since the acquisition of Jena Tec in 2012 by Kuroda Precision Industries Ltd of Japan, the company has harmonised and defined its global manufacturing and logistic supply chains to support future growth. Formerly separate brand names of “Kuroda” and “Jena Tec” are now combined as “Kuroda Jena Tec” and registered as a trademark globally. The harmony of the Kuroda Jena Tec brand symbolises the best of Japanese and German quality, accuracy and precision. The Kuroda Jena Tec ballscrew product line up is now one of the most comprehensive in the market providing from diameter 5mm up to diameter 125mm. Manufactured in both JIS and DIN standards, the company is able to offer customised and standard ballscrews in a range of diameter and pitch configurations in rolled, whirled and ground quality. All market sectors can now be adequately serviced by the Kuroda Jena Tec product range including machine tool, automation, PCB, Semi-conductor, packaging, handling, etc. High lead capability is a particular strength of the Japanese product range with many attractive features including maintenance free lubseal options and BTA end adaptors reducing delivery time. Since acquisition, Kuroda Jena Tec has expanded its market sectors and has grown sales through its enhanced sales network into mainland Europe and North America. Expansion has seen investment in new end machining capability in North America as well as significant stock investment in Europe and USA to allow Kuroda Jena Tec to shorten delivery times to same day in many cases. For more information and to find your local sales office or authorised distributor go to

Friday, January 30, 2015

Precision Air Bearing Rotary Table Installation Promotes New Interest In Oceanographic Research

Following the successful installation of a 1.4m air bearing rotary table, Kuroda Jena Tec has seen a significant increase in enquiries and demand for repeat and similar air bearing rotary tables within Europe from oceanographic related research institutes. Within 2014, Kuroda Jena Tec supplied a custom designed Air Bearing Rotary Table (ABRT) to the Institute de Recherche sur le Phenomenes Hors Equilibre located in Marseille, France. . The brief was demanding in that a non-symmetrical load of 1,000kg needed to rotate at 90rpm with a speed variation of +/-0.1%. Due to the specific nature of the research by IRPHE, a smooth rotational performance was necessary without vibration hence the choice of an air bearing in the horizontal plane that allowed minimal effects from vibration caused by conventional bearings combined with a dampening low noise belt drive and servo motor system.
Due to the incumbent load being a tank of seawater, special consideration was needed for corrosion protection and electrical insulation with the requirement to install an 18 channel slip ring for power and control instrumentation. This was necessary to allow IRPHE to mount various monitoring devices on the table surface while the table rotated. For a video link of the rotary table click here. Provided with a complete software package and customised software, the unit was able to provide custom software programmes for various planned testing including defined ramp up, ramp down, specific cycles at programmed speed and the option for oscillation cycles within a defined percentage of rotational speed. Kuroda Jena Tec provides standardised and customised air bearing rotary tables from 500mm to 2m in diameter for a variety of machining, testing and inspection applications. For more specific information please refer to Additionally Kuroda Jena Tec has extensive experience in the field of high precision linear and rotary motion components and sub-assemblies including ballscrews, linear modules, linear actuators, motorised spindles and rotary table. For more information please go to

Long Reach Grinding Spindles Provide Over 1000mm Internal Bore Grinding Capability

Kuroda Jena Tec is a designer and developer of customised motorised grinding spindles and has found increased demand for modular grinding heads for internal bore requirements in the field of grinding of aerospace landing gear components, gas turbine stators, oil and gas valve components and many other applications in heavy industry. Most recently Kuroda Jena Tec’s UK facility designed and supplied an internal bore grinding motorised spindle unit to fit to a large OEM gas turbine producer for fitting to an existing manual turning machine to allow the customer to quickly grind an internal bore of a stator component simply and easily using standard production equipment.
Following the success of this application the OEM subsequently purchased new CNC turning equipment from a specialist machine tool producer and specified Kuroda Jena Tec’s spindle for the same application. With potential bore depths up to 1500mm and motor speeds up to 20,000rpm Kuroda Jena Tec has extensive knowledge of grinding technology and know how. For more information please go to