Machine element or mechanism – Gearing – Directly cooperating gears
Reexamination Certificate
2001-07-30
2002-12-31
Joyce, William C (Department: 3682)
Machine element or mechanism
Gearing
Directly cooperating gears
C074S424880
Reexamination Certificate
active
06499374
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ball screw used in a feed system of machine tools, general industrial machinery and the like. In particular, the invention relates to a ball screw suitable for high-speed feed motion.
2. Description of the Related Art
In general, a ball screw comprises a screw shaft having a thread groove on outer peripheral surface, a ball nut having a thread groove on inner peripheral surface opposed to the thread groove of the screw shaft and a plurality of balls to be inserted between the two thread grooves and recirculated along a ball circulation passage provided in the ball nut. So as to form ball circulation passage, return tube type, insert piece type, end cap type or guide plate type is adopted. In the ball screw used in the application for the positioning with high accuracy, a ball screw is held under a preload so that the axial clearance in the ball screw is eliminated and the rigidity of the ball screw may become larger with the lesser elastic displacement under the axial load. In addition, the two thread grooves are formed generally in form of gothic arc. The methods to apply preloading are roughly divided to: fixed position preloading and fixed pressure preloading (to apply tensile preloading with a spring between two ball nuts). As the fixed position pre-loading, over-sized ball preloading method or lead-shift preloading method is adopted in case of single nut. In case of double-nut, tensile preloading method to insert a thick spacer to match the preloading amount between the two ball nuts or compression preloading method to insert a thin spacer to match the preloading amount is adopted. (For instance, see: Minoru Izawa: “Applied Technique of Ball Screw”, 1st edition, published by Kogyo Chosakai, Ltd. (May 20, 1993), pp.18-21 and pp.49-51.)
In such a type of ball screw, chromium-molybdenum steel SCM415H or SCM420 H (JIS G 4105) is used as the material for the screw shaft and the ball nut. Surface hardness is held in HRC 58-62 (Hv 650-740) by carburizing and quenching and by tempering. As the ball to bear the axial load, steel balls for ball bearings (JIS B 1501) made of high carbon chromium bearing steel SUJ2 (JIS G 4805) is used. (Hereinafter, the steel balls for ball bearings is referred as “steel balls”.) When preloading or pressurization is applied on the ball screw, spacer steel balls with diameter of several tens of &mgr;m smaller than the diameter of steel balls are often disposed at a rate of every one ball, every two balls, or every three balls with the purpose of suppressing the increase of friction torque caused by jamming of steel balls against each other. Also, the screw shaft made of carbon steel AISI 4150H or S55C (JIS G 4051) and processed by induction hardening is often used.
In the ball screw with “only steel balls” inserted in it, there have been problems as given below when high-speed feeding is performed at 60 m/min. or more, for instance.
{circle around (1)} Heat generation is increased, and this induces elongation of the screw shaft due to temperature increase. The elongation of the screw shaft leads to poor positioning accuracy.
{circle around (2)} When preloading is applied on the ball screw, the pre-load value increases due to thermal expansion of the steel balls or centrifugal force acting upon the steel balls when the screw shaft is rotated at high speed. The increase of the preload value leads to further increase of heat generation. That is, elongation of the screw shaft is increased, and this results in still poorer positioning accuracy.
{circle around (3)} Noise during ball screw operation is increased.
{circle around (4)} In case of the return tube type ball screw:
(i) Balls (steel balls or ceramic balls as to be described later) are separated from thread grooves on the screw shaft and these balls hit the tongue of the return tube when the balls enter the return tube. The increased hitting force of steel balls may repeatedly exerts action on the tongue, and the tongue may be damaged in many cases. (For instance, see “Applied Technique of Ball Screw”, ibid., pp.117-121.)
(ii) In the ball screw, a certain fluctuation or variation occurs in the circulating movement of balls due to deviation caused in fabrication and assembling stages. For this reason, the balls discharged from the return tube hit a portion near the land (cylindrical outer periphery of the screw shaft) of the thread groove of the screw shaft, and the balls are then caught in the thread grooves. The increased collision force of the steel balls repeatedly exerts action on a portion near the land and this results in damage on the thread grooves. As a result, practical service life of the ball screw is shortened.
As one of the means to solve the above problems, ceramic balls made of silicon nitride (Si
3
N
4
) with lower density and lower linear expansion coefficient than steel balls as shown in Table 1 (hereinafter referred as “ceramic balls”) are used instead of the steel balls.
TABLE 1
Characteristic values of
ceramic ball and steel ball
Ceramic ball
Steel ball
(Si
3
N
4
)
(SUJ2)
Density [g/cm
3
]
3.2
7.8
Hardness [Hv]
1700-1800
700-800
Linear expansion
3.2
12.5
coefficient [×10
−6
/° C.]
Modulus of longitudinal
310
210
elasticity [GPa]
Poisson's ratio
0.25
0.3
Thermal conductivity
0.07
0.1
[cal/cm · s · ° C.]
However, in the ball screw with “only ceramic balls” inserted in it, there have been the following problems:
{circle around (1)} When preloading or pressurization is applied on the ball screw and high-speed feeding is performed, temperature increase is relatively lower compared with the case where only the steel balls are inserted. However, axial clearance is apt to occur and then preload is lost, which will result in poorer positioning accuracy. The reason to cause the axial clearance is that the diameter of ceramic ball is smaller compared with sizes of the screw shaft and the ball nut made of steel (linear expansion coefficient: 11.9×10
−6
/°C.), and linear expansion coefficient is also lower. As a result, it is more likely to be subjected to the influence of temperature increase.
{circle around (2)} Compared with the ball screw with only the steel balls inserted, the screw shaft has generally shorter service life. This is attributed to the fact that, if steel ball and ceramic ball have the same diameter, when the same axial load (i.e. the same magnitude of rolling element load) is applied, contact surface area of the ceramic balls and the two thread grooves is smaller than that of the steel balls. As a result, high contact stress is caused on contact surface, and this leads to shorter service life of the thread grooves (according to the elastic contact theory by Hertz).
{circle around (3)} When a rotating screw shaft is stopped and is rotated in reversed direction, shock load is applied on contact surface of balls (steel balls or ceramic balls) and the two thread grooves. (The higher the rotating speed of the screw shaft is, the higher the shock load is.) Ceramic balls are harder than steel balls and have higher modulus of longitudinal elasticity than steel balls (See Table 1 above), and indentation is more likely to occur on the thread grooves due to the shock load. As a result, noise during ball screw operation is increased, and service life of the thread grooves is decreased due to indentation. Thus, ceramic balls are inferior to steel balls in the shock resistance of the thread grooves.
{circle around (4)} Ceramic balls are very expensive in cost compared with steel balls. For instance, in case of a ball with nominal diameter of ¼ inch (6.35 mm), market price of steel ball is about 0.8 yen/piece, while that of ceramic ball is 20 yen or more/piece.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a ball screw, by which it is possible to overcome the above problems in the conventional type ball screw with “only steel balls” or “only ceramic balls” inserted in it and which
Howson and Howson
Joyce William C
Tsubaki Nakashima Co., Ltd.
LandOfFree
Ball screw with alternately disposed steel balls and ceramic... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Ball screw with alternately disposed steel balls and ceramic..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Ball screw with alternately disposed steel balls and ceramic... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2991328