Double Enveloping Technology
Double-enveloping worm gearing provides several distinct advantages over cylindrical worm gearing, including increased torque throughput, improved accuracy, and extended life.
The concept of worm gearing originated in antiquity. Archimedes is generally acknowledged as the inventor of the worm gear, using a screw to rotate a toothed wheel. Leonardo Da Vinci envisioned the double-enveloping worm gear nearly 500 years ago. Although Da Vinci came up with the concept for double-enveloping worm gearing, it was Samuel Cone who developed efficient techniques to manufacture the double-enveloping worm gear in the late 1920's.
Different Types of Worm Gears
There are three different types of worm gearing: non-throated, single-throated, and double-throated (see Figure 1).
In non-throated worm gearing, neither the worm nor the driven gear is throated. In single-throated worm gearing, one element (usually the driven gear) is throated. In double-throated worm gearing, both the driven gear and the worm are throated.
Improved Contact Patterns
Double-enveloping worm gearing possesses several key advantages over other types of worm gearing. In a cylindrical worm gearset, only one to two gear teeth are in contact with the worm.
In a double-enveloping worm gearset, three to eleven gear teeth are typically in contact with the worm, depending upon the ratio. The increased number of driven gear teeth that are in contact with the worm significantly increases torque capacity, and also raises shock load resistance.
In addition to increasing the number of driven gear teeth in contact with the worm, double-enveloping worm gearing also increases the contact area on each gear tooth. The actual areas of instantaneous contact between the worm threads and the driven gear tooth are lines. These lines of contact move across the face of the gear tooth as it progresses through its total time of mesh with the worm. The lines of contact in double-enveloping worm gearing are configured to increase the power transmission capability and reduce the stress on each gear tooth.
Figure 2 shows a cylindrical worm gearset where two gear teeth are in mesh with the worm. Each meshed gear tooth has a single line of contact extending across half of its width. As a gear tooth is rotated through its arc of contact with the worm, the line of contact sweeps from the tip of the gear tooth to its root. The line of contact is approximately aligned with the worm thread's sliding direction.
Analysis of Lines of Contact for Cylindrical Worm Gearing
As the gear tooth is rotated through its arc of contact with the worm, the line of contact sweeps from the tip of the gear tooth to its root.
Improved Contact Patterns
Figure 3 shows the same analysis for Cone Drive double-enveloping worm gearing. Between three and eleven gear teeth (five in this example) are in mesh with the worm at any moment. During a significant portion of the mesh cycle, the geometry of the double-enveloping worm gear creates two lines of contact on each gear tooth instead of one. As a gear tooth is rotated through its arc of contact with the worm, one line of contact is maintained at the center of the tooth. The other contact line sweeps from the left (entering) side of the tooth toward the center. During the end of the tooth's movement through the contact arc (positions 4 and 5), the two lines of contact converge into a single line at the center of the tooth. The lines of contact are roughly perpendicular to the worm thread sliding action.
Analysis of Lines of Contact for Double-Enveloping Worm Gearing
As the gear tooth is rotated through its arc of contact with the worm, one line of contact is maintained at the center of the tooth, while the other line sweeps from the edge to the center of the tooth. The two contact lines converge into one near the end of the mesh cycle.
The Double-Enveloping Technology Advantage
The benefits of this double-enveloping design are dramatic. First, the total load is divided among more individual gear teeth, and the load is further divided where teeth support two lines of contact. This superior load distribution greatly increases load carrying capacity. Second, the improved torque throughput allows a smaller reducer to produce the same amount of torque, resulting in size and weight savings.
Double-enveloping worm gearing can carry loads that would require much larger and heavier cylindrical worm gearing.
Superior Lubrication Characteristics
The increased number of gear teeth in contact with the worm threads reduces the load at any given point on each gear tooth, improving lubrication. In addition, the two lines of contact on each gear tooth that result from the improved geometry of double-enveloping worm gearing further share the load and improve lubrication. And because the lines of contact are approximately perpendicular to the worm thread sliding action, lubricant is more readily introduced into the space between the gear tooth and the worm thread face. With this perpendicular direction of introduction, the lubricant has only a very short distance to travel underneath the width of the line of contact. Conventional cylindrical worm gearing, on the other hand, features a worm thread sliding action parallel to the direction of the line of contact. This parallel movement increases the likelihood of expelling the lubricant and allowing direct metal-to-metal contact.
The Double-Enveloping Design
The design of double-enveloping worm gearing is based on a unique tooth form concept. Instead of involute or other curved tooth forms, most double-enveloping gears have straight-sided forms on both gear teeth and worm threads with this form tangent to a common base circle.
Most double-enveloping worm gearing has straight-sided forms on both gear teeth and worm threads, with this form tangent to a common base circle.
Experience over many years has established the practical design proportions for helix angles, pressure angles, number of teeth in gear, gear widths, relationship of worm pitch diameter to worm root diameter, tooth thickness, and backlash. Most of these parameters are outlined in AGMA/ANSI specification 6030-C87, which contains design formulae as well as tables of recommended proportions. These parameters have been used to establish a standard line of double-enveloping gearing and custom gearing for specific applications.
Double-enveloping gearing is typically designed with a 55% to 45% tooth thickness ratio, with the gear 55% of the circular pitch and the worm 45% of the circular pitch. This gives a balanced design since the worm, which is made of alloy steel, is the stronger member (120,000 psi yield) and the gear, made from tin-bronze, is the limiting member (25,000 psi yield). By making the gear tooth thickness greater than the worm thread thickness, the two members are more nearly equal in relative strength.
Backlash in gears is generally defined as the play between mating teeth. For purposes of measurement and calculation, backlash is defined as the amount by which a gear tooth space exceeds the thickness of the engaging worm thread on the pitch circle of the gear. Or, stated another way, backlash is the movement at the pitch line of the gear (in millimeters) or as rotational movement of the gear (in degrees or arc minutes) with the worm locked.
Double-enveloping worm gearing provides very low backlash when compared with cylindrical worm gearing. Because the amount of backlash is lower, double-enveloping worm gearing is inherently more accurate than cylindrical worm gearing. The improved accuracy makes double-enveloping worm gearing especially well suited for applications such as metal cutting and forming machinery, and printing and packaging machinery where precision motion control is critical.
Applications for Double-Enveloping Worm Gearing
Double-enveloping worm gears are used in a number of industries to achieve precise positioning and increased throughput. Some of these applications include:
Metal cutting and forming machinery- Where metal cutting, drilling, and forming operations require workpiece positioning accuracy, the low backlash of double-enveloping worm gearing keeps positioning accuracy high, even with heavy workpieces and repeated starting and stopping motions.
Mining and construction equipment- In applications where very high torque capacity and shock loading resistance are mandatory, double-enveloping worm drives provide rugged reliability. Because they offer high ratio reduction in a single stage, these drives provide a compact, low profile that is ideal for many types of mining equipment like feeder breakers, shuttles, and conveyors. Double-enveloping worm drives are also an excellent choice for rock crushers, which require high shock resistance.
Printing and packaging machinery- With very low backlash, double-enveloping worm gearing helps printing press rolls maintain precise print registration at very high speeds. The gearing's high shock load capability allows extremely rapid speed adjustments during operation. And since the gearing has low inertia due to its small size, large, complex presses and coaters can be started or stopped more quickly than those using heavier, multi-stage gearing.
Paper and plastic converting machinery-Double-enveloping worm gearing provides high torque capacity and continuous, reliable service in roll drives, winders, and slitters. Theses drives are designed to handle the very high torque requirements of roll turnover stands and turret indexers.
Construction equipment- Double-enveloping worm gearing provides the torque capacity and shock resistance required to keep construction machinery operating reliably. This type of gearing can be used for power transmission systems subject to heavy, changing loads or where space is at a premium. In drum hoists, winches and cranes, double-enveloping worm gearing delivers high torque throughput and smooth operation.
Iron and steel production equipment- With very high shock load resistance, double-enveloping worm gearing performs extremely well in slab pushers and side adjusters. Double-enveloping worm gearing is an ideal choice for screwdowns, where high torque loads and frequent adjustments reduce conventional gearset life. It is also used in furnace tilt drives, roll drives, and slitters, where high load capacity is required. Crane and hoist capacities can be increased with the high torque throughput and compact size of double-enveloping gear drives.