Thermal compensation belt drive tensioner

Details Thermal compensation belt drive tensioner Thermal compensation belt drive tensioner

2000-05-10

2002-11-26

Hannon, Thomas R. (Department: 3682)

Endless belt power transmission systems or components

Means for adjusting belt tension or for shifting belt,...

Reexamination Certificate

active

06485383

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to belt drives, more specifically to hard copy apparatus having a scanning carriage for translating writing instruments across print media and, more particularly, to a thermal compensation belt drive tensioner for a scanning ink-jet printer carriage.
2. Description of Related Art
The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed for example, in various articles in the assignee's
Hewlett
-
Packard Journal,
Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No.1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in
Output Hardcopy [sic] Devices,
chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press. San Diego, 1988).
FIG. 1
depicts a hard copy apparatus, in this exemplary embodiment a computer peripheral, ink-jet printer,
101
. A housing
103
encloses the electrical and mechanical operating mechanisms of the printer
101
. Operation is administrated by an electronic controller
102
(usually a microprocessor or application specific integrated circuit (“ASIC”) controlled printed circuit board) connected by appropriate cabling to a computer (not shown). It is well known to program and execute imaging, printing, print media handling, control functions and logic with firmware or software instructions for conventional or general purpose microprocessors or with ASIC's. Cut-sheet print media
105
, loaded by the end-user onto an input tray
120
, is fed by a suitable paper-path transport mechanism (not shown) to an internal printing station, or “print zone,”
123
where graphical images or alphanumeric text is created. A carriage
109
, selectively positionable on a rod
111
, scans the print medium. An encoder subsystem
113
is provided for keeping track of the position of the carriage
109
at any given time. A set of individual ink-jet pens, or print cartridges,
115
X is mounted in the carriage
109
(generally, in a full color system, inks for the subtractive primary colors, cyan (X=C), yellow (X=Y), magenta (X=M) and true black (X=K) are provided; in some implementations an ink-fixer chemical (X=F) is also used). An associated set of replaceable or refillable ink reservoirs
117
X is coupled to the pen set by ink conduits
119
. Once a printed page is completed, the print medium is ejected onto an output tray
121
. The carriage scanning axis is conventionally designated the x-axis, the print media transit axis is designated the y-axis, and the printhead firing direction is designated the z-axis. For convenience in describing the art and the present invention, all types of ink-jet hard copy apparatus are sometimes hereinafter referred to as “printers;” all types, sizes, and compositions of print media are also referred to as “paper;” all compositions of colorants are sometimes referred to as “ink;” and all embodiments of an ink-jet writing instruments are sometimes hereinafter simply referred to as a “open;” no limitation on the scope of the invention is intended nor should any be implied.
Generally, a belt drive subsystem can be used to mount and selectively move the carriage
109
for scanning bidirectionally across the print zone
123
. Instantaneous positioning of the printhead to the print medium is critical to prevent a printing error and ensure throughput performance. Belt tension is an important parameter and is varied dependent primarily on the drive loads required, margin desired, and belt-pulley design.
FIG. 2
(PRIOR ART) illustrates a belt
100
connected to a reversible motor
300
and a pulley
500
, both of which are usually affixed to a printer housing framework (not shown). A pulley axle
700
fits slidingly in slots
900
in a mounting frame
110
. Assembled, the belt
100
extends through an aperture
130
in the frame
110
. The pen carriage
109
(not seen in this view) rides on a guide rod
111
as in FIG.
1
and is attached to the belt
100
. Since the pulley axle
700
makes a sliding fit and the belt must be long enough to reach beyond the end
170
of the mount
110
to encircle the pulley
500
before it is fit into the slots
900
, a spring loaded tensioner
190
is used to achieve the proper post-assembly tension. After the pulley
500
is fit into the slots
900
, the tensioner
190
is inserted such that the axle
700
will ride on a tensioner surface
210
. An extension table
230
, with a tensioning spring
250
surrounding it, is rotatingly slipped into slot
270
of the mount
110
. By designing to a close tolerance, the tensioner surface
210
will pull the pulley outward along the slots
900
just until the belt
100
is properly tensioned. Another spring loaded tensioner is shown in U.S. Pat. No. 4,761,154, filed by Beauchamp et al. for a BELT TENSIONER (assigned to the common assignee herein and incorporated herein by reference).
Another prior art design, that has a lower belt tension requirement over the spring loaded design, is the standard fixed center tensioner that includes a spring loaded belt tensioner that provides initial belt tension and a locking mechanism to fix the tensioner to a predetermined setting. As the belt stretches under load and during thermal and humidity excursions, the hardware expands and contracts, resulting in changing pulley center-to-center spacing. The result is a change in belt tension from the initial setting.
In such solutions, as the belt stretches under load and during thermal and humidity excursions, the hardware can also expand and contract, resulting in a spring deflection and, therefore, a belt tension change. Such systems are designed with the lowest possible spring rate so that such a tension change is minimized. However, at higher acceleration and deceleration loading and higher scanning speeds which increase printing throughput, belt tensions must be increased to prevent belt slip at the motor and pulley interfaces. Moreover, for an ink-jet implementation, due to the advancement in pen design and increasing the number of printheads on-axis (or, for disposable or refillable print cartridges having a self-contained, on-axis, ink supply where the cartridge size is increasing to meet the demand for full bleed printing (e.g., photographic reproductions)), the total carriage weight increases. Increasing belt tension increases loading on the motor axle bearings. One solution is to add precision bearings to the motor shaft, but only at a significant cost impact on manufacturing. During temperature and humidity excursions, the material will experience an expansion or contraction (“EC”) proportional to the respective material coefficient of thermal expansion (“CTE”). Note that CTE can be a positive or negative value. Depending upon the materials used in fabricating the mechanism, differing material CTE may generate a belt tension increase or decrease during temperature excursions.
There is a need for a method and apparatus to maintain a substantially constant belt tension during temperature changes, There is a need for a method and apparatus to run relatively low belt tensions in high speed printers to increase motor life. There is also a need for a low cost solution.
SUMMARY OF THE INVENTION
In its basic aspects, the present invention provides a belt tensioner device, including: an adjustably mountable frame for mounting to a chassis; a pulley fork biasingly mounted to the frame; and a belt pulley mounted between the frame and the fork wherein the frame and fork are co-associated and fabricated of materials each having a complementary CTE to compensate for temperature excursions affecting belt tension.
In another basic aspect, the present invention provides a belt tensioner for an apparatus having

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