Drive for backlash-free conversion of motion

A drive mechanism is provided for backlash-free conversion of a rotary motion into a linear motion with the aid of a toothed belt and of a drive gear, in particular for a quick and reproducible positioning of tools or tool tables. At least one gear rack adjustable in one direction is present. Arcuate guides are provided at the two sides of the drive gear. The arcuate guides maintain the toothed belt in such engagement with the gear rack that one section receives the load and only a short, defined section of the toothed belt is disengaged from the teeth. Furthermore, the toothed belt is provided as an endless loop.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a drive device for backlash-free 
conversion of rotary motion into linear motion by use of a toothed belt 
and of a drive gear positioned to engage the two belts. 
2. Brief Description of the Background of the Invention Including Prior Art 
High requirements are associated with the capacity and accuracy of 
measurement and tool machines where slides, supports, tools or tool tables 
perform linear motions and with machines with cross support slides that 
perform simultaneous motions in the directions of two coordinates. In 
these cases, the linear motions are not only to be performed rapidly, that 
is, with large acceleration and deceleration values, but also with highest 
accuracy in order to maintain desired production tolerances. Finally, the 
motion of these machine parts is to be free of slippage, to be free of 
backlash and play, and to be free of vibrations in order to allow mass 
production under reproducible conditions with the aid of numeric controls. 
The requirements lead in practical terms, because of the high loading of 
the component parts of the linear drive, to undesired wear and tear, which 
result in the long run in inaccuracies and scattering during the numeric 
positioning of the slides or tools. The known and usually employed drive 
systems for the performance of linear adjustment motion, which, for 
example, comprise threaded spindles, screws, recirculating ball nut and 
screw drives, gear racks, hydraulic or pneumatic pistons or toothed belts. 
These systems and devices are all applied because of their properties and, 
in fact, as is understandable, always where based on the requirements 
their favorable properties are used to especial advantage. 
However, when the question arises of providing linear drive where all 
conditions are fulfilled to a very high degree, for example, in case of a 
large adjustment path under large adjustment speed, for example, 1.0 
m/sec, then it is difficult to obtain a backlash-free, vibration-free and 
accurate positioning. In particular, this is difficult especially in 
continuously repetitive runs. Thus, for practical purposes, all known 
drive systems are eliminated despite their specific advantages because 
their application under the conditions described increases their 
disadvantages to such an extent that such linear drive for an optimum 
solution for the requirements is hopeless from the beginning. 
Systems of this kind comprise at least one gear rack running in an 
adjustment direction. The two sides of the drive gear are provided with 
arcuate guides, which maintain the toothed belt in such engagement with 
the gear rack that a section accepts the load and only a short defined 
section of toothed belt remains out of engagement with the teeth and where 
furthermore the toothed belt is provided as an endless loop. Such a drive 
is known for example, in principle, from the German Pat. No. 2910373. 
A device is taught in U.S. Pat. No. 3,824,871 where a container is placed 
in a back and forth motion by means of a chain drive actuated by chain 
gears. This is employed for a relatively uniform distribution of spread 
bulk materials in a storage space. Precision motion and precision 
positioning cannot be achieved with this known device. In particular, 
during a change in the direction of motion in this device, the required 
play between the elements engaging each other is substantially 
interfering. 
The drive device operating with a toothed belt taught in U.S. Pat. No. 
3,850,043 comprises a separate adjustment provision for eliminating the 
backlash of the toothed belt. The backlash between the toothed belt and 
the gear rack however cannot be eliminated with this additional element. 
It is in addition, a disadvantage of this device that depending on the 
length of the toothed belt, that is, depending on the position of the 
slide, there occur different changes in length of the toothed belt under 
the same force action. Thus inaccuracies in positioning result, which 
cannot be tolerated in case of a numerically controlled machine tool. 
SUMMARY OF THE INVENTION 
1. Purposes of the Invention 
It is an object of the present invention to provide a drive device for the 
backlash-free conversion of rotation into linear motion, which allows the 
movement of tools, slides or other elements to be positioned up to very 
large distances without backlash and slippage. 
It is another object of the present invention to provide a drive device 
with low mass inertia, which can be linearly moved with high positioning 
accuracy. 
It is yet another object of the present invention to provide a drive device 
that is particularly easy to service and to maintain and is reliable in 
its ability for position control. 
These and other objects and advantages of the present invention will become 
evident from the description which follows. 
2. Brief Description of the Invention 
The present invention provides a drive transmission for backlash-free 
mechanical energy transfer between rotational and linear motion comprising 
a first deflection roller disposed in parallel to a second deflection 
roller and a drive gear whose axis is disposed about in parallel to the 
direction of the deflection rollers and which is at a closer distance to 
each drive than the drives are to each other. An endless toothed belt 
surrounds with its back side the surface of each of the two rollers in a 
force transmitting way at least over an angle of about 180 degrees and 
engages the drive gear with its front side in such a way that its back 
side in the area of the contact with the drive gear and most remote from 
the two rollers it is disposed between a plane spanned by the axes of the 
two rollers and a tangential plane spanned by the two rollers at a 
location going in the direction from the gear axis to the contacting of 
the toothed belt with the drive gear. A toothed rack is disposed 
relatively close to and has its teeth directed toward the above cited 
tangential plane to engage the toothed belt in such a way that only a 
relatively small section of the toothed belt is not provided with a 
positive locking or force transmitting support. 
The toothed rack can run quickly and repeatably in an adjustment direction 
of a tool and is in constant engagement with the toothed belt. Arcuate 
guides can maintain the gear belt in such engagement with the toothed gear 
that a certain section receives the load generated by the toothed rack and 
that only certain short second sections are without engagement with the 
teeth or without back support. 
The drive gear can be limited in axial direction at least on one side by a 
cylinder roller that has a slightly larger diameter than an addendum 
circle of the drive gear and that can roll on one of the deflection 
rollers. 
The axes of the deflection rollers can be connected to each other by a 
frame, where the frame can be rigid, but movable to different height 
levels. A hinged frame can be employed to allow for changing the distances 
between the axes. The axis of the drive gear can be connected to a frame. 
The engagement region between the toothed belt and the toothed rack can 
include an additional press-on stage, which presses the toothed belt from 
its backside into the teeth of the toothed rack. The press-on stage can be 
provided with air nozzles providing for an air cushion at its contacting 
surface. Contact pressure generating rollers can be employed for 
maintaining the toothed belt disposed at the toothed rack. 
The teeth of the toothed belt can have small gearing angles. The gear belt 
can be provided with teeth having essentially convex semicircular 
cross-sections. 
The diameter of the gear wheel can be from about 0.5 to 1 times the 
diameter of the deflection rollers. 
Another aspect of the present invention provides a drive transmission for 
backlash-free mechanical energy transfer between rotational and linear 
motion, which has a gear drive with a rotation axis next to a first 
deflection roller with a rotation axis. A second deflection roller with a 
rotation axis is disposed next to the gear drive such that the angle 
between lines providing a nearest connection between the gear drive axis 
and the first roller axis and the gear drive axis and the second roller 
axis form an obtuse angle. An endless toothed belt runs with its back side 
in rolling contact with the respective rollers, and one belt connection 
between the rollers engages the teeth of the gear drive with its front 
side such that the above recited lines approximately correspond to a 
transition of engagement of the toothed belt between the gear drive and a 
respective roller. The second belt connection between the rollers runs 
with the back side of the belt approximately along a tangential plane 
jointly defined by the surfaces of the two rollers. A gear rack engages 
the front side of the toothed belt in an area corresponding to the belt's 
motion along said tangential plane. 
Means can be provided for pressing the toothed belt against the gear rack 
in the area of the tangential plane between the surfaces of the rollers 
and can, for example, be provided by an air flow exiting from nozzles or 
by support rollers having an axis running approximately in parallel with 
the axis of the deflection rollers. 
Toothed belts, in particular, toothed belts made from plastic, can be 
produced with high accuracy with regard to the shape and pitch of the 
teeth. These toothed belts are sufficiently flexible, they are stable 
against wear, are low-noise devices and have a favorable degree of 
efficiency. They are produced in an endless loop and can be provided both 
with a steel wire core as well as with a glass fiber reinforcement. 
The novel features which are considered as characteristic for the invention 
are set forth in the appended claims. The invention itself, however, both 
as to its construction and its method of operation, together with 
additional objects and advantages thereof, will be best understood from 
the following description of specific embodiments when read in connection 
with the accompanying drawing.

DESCRIPTION OF INVENTION AND PREFERRED EMBODIMENT 
The present invention provides a drive device for backlash-free conversion 
of a rotary motion into a linear motion with the aid of a toothed belt 11 
and a drive gear 14 engaging the toothed belt 11, where at least one gear 
rack 10 is present running in a desired motion direction. The gear rack 10 
is constantly engaging the toothed belt 11. Arcuate guides are provided on 
two sides of the drive gear 14 to maintain the toothed belt 11 in such 
engagement with the gear rack 10 that a certain section B assumes the load 
and only a short defined section D of the toothed belt 11 remains without 
engagement with a guide or counter teeth. The endless loop of the toothed 
belt 11 force transmittingly surrounds two deflection rollers 12, 13 with 
its back side. A drive gear 14 engages the toothed belt 11 from the side 
of the loop remote from the gear rack 10 such that only a negligibly small 
section D of the toothed belt 11 is not provided with a positive locking 
or force-transmitting support. 
A pressurizing means can be disposed next to the toothed belt in the 
engagement area between belt and gear rack to press the teeth of the belt 
into the teeth of the gear rack. Contact rollers can be disposed against 
the rear side of the toothed belt for maintaining the belt against the 
gear rack. 
The drive device illustrated in FIG. 1 comprises substantially a gear rack 
10, an endless toothed belt 11, two deflection rollers 12 and 13, as well 
as a drive gear 14. These elements are driven by an electric motor in a 
conventional way. The drive belt 11 is pressed via deflection rollers 12 
and 13 into the teeth of the gear rack 10. Thereby a sufficient engagement 
of the toothed belt 11 into the gear rack 10 is ensured over a length B. 
The engagement of the drive gear wheel 14 into the toothed belt 11 occurs 
in the region A. A further region designated as C provides that the 
toothed belt 11 is resting force transmittingly on the circumference of 
the deflection roller 12 or, respectively, 13. Only in the region 
designated D is a free stretching of the toothed belt 11 possible. 
However, this region, as can be clearly recognized from the drawing, can 
be reduced to zero for practical purposes such that free stretching of the 
toothed belt 11 can be excluded. In order to achieve this, it is only 
required that the drive gear wheel 14 is disposed relative to the 
deflection rollers 12 and 13 such that it comes to rest very deeply 
between the deflection rollers 12 and 13. In the region C, a slippage is 
only then possible if the rest friction between the back of the toothed 
belt 11 and the deflection roller 12 or, respectively, 13 is surpassed. 
This however can be excluded under normal operation by suitable selection 
of the frictional relationship. Only with occurrence of jolts is a short 
time duration slippage possible, which, however, leads to a reduction of 
the jolt energy. Following this, however, the desired precision conditions 
are again immediately present. Even the short term loss of coordination 
between the position of the drive gear wheel 14 and the position of the 
total drive device is restored immediately after the decreasing of the 
jolt load. 
The drive means illustrated in FIG. 1 is associated with the great 
advantage that not only can the toothed belt together with the deflection 
rollers 12 and 13 be lifted off the gear rack 10 with no problem and 
because of this, several of such drive provisions can be operated on a 
single gear rack system, but also that the proper drive, along with the 
drive gear wheel 14, can be separated from the remaining parts of the 
drive without great demounting labor. This can be clearly recognized from 
FIG. 1. Of course, systems with several tracks can also be employed with 
the drive device described above. For example, it is possible without 
further problems to operate a system with two or more gear racks 10 and a 
corresponding number of toothed belt drives. 
Possible variations of the above described construction in principle can be 
obtained by providing a pressure means 15 in the engagement region B 
between the gear rack 10 and the toothed belt 11 in the area between the 
deflection rollers 12 and 13. The pressure means presses the toothed belt 
11 from the back side against the gear rack 10. In order to decrease 
friction in this context, a plurality of air exit nozzles can be provided 
in the contact surface of the pressure means 15. These air exit nozzles 
set up an air cushion and thus substantially reduce the frictional effect. 
The pressure means 15 can also be substituted for completely or in part by 
contact rollers 15'. Thus, particularly in the running direction of the 
toothed belt 11, such contact rollers could be provided on both sides of 
the pressure means 15. 
The axes 16 and 17 of the deflection rollers 13 or, respectively 12, can be 
connected to each other via a frame 21' or linkage. A rigid frame can be 
employed, but a hinged linkage can also be used. For example, a hinged 
linkage can be constructed from the hinges 19 and 20, where these hinges 
can also be directly or indirectly connected with the axis 18 of the drive 
gear wheel 14. A loading of the axis 18 of the drive gear wheel 14 with a 
perpendicular force thereby effects a slight pressing apart of the axes 16 
and 17 of the deflection rollers 12 and 13 as well as a deeper penetration 
of the drive gear wheel 14 into the intermediate space between the 
deflection rollers 12 and 13. 
In order to have the device free from play, it is required that an optimum 
matching of the shape of the toothed belt 11 with regard to the gear rack 
10 be provided. This is achieved by a perpendicular movability of the 
guide of the toothed belt provided in the pressing means 15. Contact 
pressure rollers or similar devices can provide a permanent concentrically 
acting compression force. Thus the full engagement region becomes 
effective independent of the position. The pressure means 15 and the 
contact pressure rollers can be disposed movable in perpendicular 
direction for example under the action of a spring force. In addition, the 
axes 16 and 17 can be disposed movable in a perpendicular direction. 
Thereby possible errors in the shape of the gear rack 10 can be balanced. 
The drive gear wheel 14 can be provided axially at least on one side with a 
cylinder roller 21. The diameter of the cylinder roller 21 can be slightly 
larger than the head circle of the drive gear wheel 14. A direct contact 
of the cylinder roller 21 with the deflection rollers 12 and 13 is 
possible upon a corresponding deep penetration of the drive gear wheel 14 
between the deflection rollers 12 and 13. Thus a limitation of the 
penetration of the drive gear wheel 14 into the intermediate space between 
the deflection rollers 12 and 13 is provided. The thus achieved direct 
drive of the deflection rollers 12 and 13 thus also activates the region C 
for the drive of the toothed belt. 
The length of the gear rack 10 can be as desired. For this purpose, it is 
composed of individual pieces of suitable length, possibly under 
maintaining of certain spaces between individual sections of the gear 
rack, such that by the selection of correspondingly large or small 
distance, even production errors in the production of the gear rack can be 
essentially balanced. 
Preferably toothed belts are employed which provide a so-called null 
interval, that is, they engage without play into the profile of the gear 
rack. However, divided gear rack profiles can also be employed, which are 
then pushed against each other such that a possibly present play and/or 
backlash becomes balanced. 
Upon use of tooth profiles with small flange angles, the lifting forces 
which would tend to make the toothed belt rise out of the gear rack 
profile can be kept very small. In particular, a tooth profile with 
semicircular tooth cross section as shown in FIG. 2 can be very 
advantages. 
The above described drive device is suitable for numerically controlled 
machines. The device can be used with a lift length which is practically 
unlimited. Because of the small masses required in the device, a high 
acceleration can be achieved with a compensation for jolts. The device 
allows a component construction where several drive devices can be 
operated on a single gear rack system. The present system permits in 
addition the bridge building mode with twin drive (portal devices) without 
further construction. The device of the present invention is easy to 
service because of the ease of exchangeability of both the complete device 
as well as the individual parts of the device, and it further is 
associated with a high degree of effectiveness and a long lifetime. 
The gear belt can have a single sided or double sided toothing in order to 
transmit circumferential forces without slippage. The belt body can 
comprise for example polyurethane or neoprene with fold fibers of steel or 
glass fiber yarn which are wound helically in the generally in standard 
length produced belts. The pull fibers provide the neutral bending plane. 
The length of the pull fibers is in general the effective length L.sub.W 
of the belt and its radius in the surrounding arc of the effective arc. 
When properly adjusted, toothed belts run without service with or without 
lubrication. With larger speeds, larger powers, larger pretensions and 
larger widths of the belt, noises are generated associated with the 
engagement of the teeth. 
If the teeth of the gear of the toothed belt are of circular shape, their 
radius can be from about one fifth to one twentieth of the radius of the 
rollers and preferably from about one eighth to one fifteenth of the 
radius of the rollers. The number of the teeth of the toothed belt 
engaging the gear rack can be from about 5 to 50 and is preferably between 
about 10 and 20. The number of teeth of the drive gear can be from about 
10 to 50 and is preferably between about 20 and 30. The diameter of the 
drive gear can be from about one half to the full diameter of the rollers. 
The angle between the line connecting the first roller with the axis of the 
drive gear and the axis of the second roller with the axis of the drive 
gear is preferably larger than 135 degrees. The position of the axis of 
the drive gear is preferably between a tangential plane of the respective 
surface of the two rollers which is relatively remote from the gear rack 
and a plane going through the axes of the two rollers. The width of the 
gear rack can be from about one eighth of the radius of a roller to one 
half of the full size of such roller and is preferably between 0.25 and 
0.5 of the radius of one of the rollers. The thickness of the belt can be 
from about one fifth to about one fiftieth and is preferably from about 
one tenth to about one twentieth of the radius of a roller. The average 
flange angle of the teeth is preferably between about 10 and 30 degrees. 
The distance between the back side of the toothed belt in the area between 
the two rollers at their closest distance is preferably between one 
sixteenth and the size of the radius of the roller and preferably from 
about one quarter to three quarters of the radius of a roller. 
It will be understood that each of the elements described above, or two or 
more together, may also find a useful application in other types of drives 
and motion conversion procedures differing from the types described above. 
While the invention has been illustrated and described as embodied in the 
context of a drive for backlash-free conversion of motion, it is not 
intended to be limited to the details shown, since various modifications 
and structural changes may be made without departing in any way from the 
spirit of the present invention. 
Without further analysis, the foregoing will so fully reveal the gist of 
the present invention that others can, by applying current knowledge, 
readily adapt it for various applications without omitting features that, 
from the standpoint of prior art, fairly constitute essential 
characteristics of the generic or specific aspects of this invention. 
What is claimed as new and desired to be protected by Letters Patent is set 
forth in the appended claims.