Worm drive

Worm gears are a category of Schraubwälzgetriebe and consist of a helical screw called that in this cross gear ( worm gear ) rotates at rotational movement.

In contrast to the rolled gear and the sliding movement is needed. This is due to the function induced relative motion of the contacting surfaces of the worm and worm wheel. This is the main reason for the low efficiency at high ratios (~ 0.45 to 0.96 ) and the most necessary cooling of the gearbox. This is also the reason that the worm gear teeth of the quietest drive.

Due to the line contact and multiple simultaneous meshing, the worm gear is characterized by a very high load capacity.

Construction and application

A worm gear consists of a fitted with one or more screw threads shaft of the screw, and it meshed helical gear, worm wheel. The axes of the two are usually offset by 90 °. Special applications for such a transmission are where in one step high reduction ratios (up to 150 ) and / or self-inhibition are required. There are left and right rising snails. Both directions are equally possible. The drive for the most applications, via the screw.

The gear ratio is the ratio of the number of teeth of the worm wheel z2 by the coefficient z1 of the screw. When driven by the screw a speed conversion takes into Slow at the drive wheel on the other hand an increase in speed is (only possible with non -self-locking pairings ).

Self-locking occurs by the sliding friction between the worm and worm wheel, but only at a high ratio, low coefficients and a pitch angle of the screw gamma < 5 °. It causes by means of a self-locking worm gear only greatly reduces the speed, but can not be greatly increased. This effect can be used with in order to dimension needed brakes on gears for elevators or hoists smaller can. For security reasons, the self-locking can not replace a brake.

Find use as a worm gear power transmission in presses, in rolling mills, in conveying engineering, in mining machines and rowing machines, as well as in precision designed as a duplex worm milling heads and rotary tables.

Other applications include actuators, for example, in more common trim resistors.

Example of the speed increasing ratio are mechanical drives of centrifuges in dairy machines, rotary knobs and aerodynamic braking or governor in gramophones and music boxes.

The worm

In general, the worm is a special form of a helical gear. The angle of the helical gear is such that a tooth multiply helically winds around the wheel axle. The tooth is referred to in this case as a gear. There are Single or multiple screw. Usual materials for screws are used or nitriding. The surface obtained by carburizing or nitriding a wear-resistant layer.

The worm

Worm gears consist predominantly of Sn bronzes ( CuZn ) and are made of a certain size for cost reasons as wheel rims, which are mounted on favorable base ( made ​​of cast steel for example ). The production takes place in the continuous casting GC for small and subordinate gears or spin casting GZ at higher strength requirements in Verzahnbereich. Bronze is because of the good emergency running properties in conjunction with steel for transmissions. At high torques to be transmitted will make use of high-strength brass and aluminum bronze. This is to ensure that the sliding speed over the Sn bronze wheels must be greatly reduced.

Species

• Cylindrical worm gear The base body of the screw has a cylindrical shape and the worm wheel corresponding to the diameter of the screw ( globoidische ) curvature. Advantage is the ease of manufacture of the screw, which is why this type is the most common and is also referred to as " standard shape ".

• Globoid worm gear Here, the screw has a globoidische form and the worm is cylindrical. Great advantages the ability to use a conventional helical gear as a worm wheel, however, the manufacturing of the screw is considerably more expensive.

• Enveloping This is a special design, be carried out at both the worm and worm wheel as globoidische body. Due to the high manufacturing costs of worm gearing, however, they are used only as individual pieces and in special cases and are not typically manufactured in series.

• OTT helical: Game Adjustable worm gear The worm is in this version of 2 parts: a shaft screw and hollow screw. By twisting the two screw parts the game between the worm and worm wheel is adjusted. The teeth are formed such that each carries only one side of the worm tooth in the sprocket.

• Duplex Worm gear: Game Adjustable worm gear Worm shaft with two different slopes / modules on the front and back edge. By axial adjustment of the auger relative to the worm stronger spirals are brought into play poor intervention. Proportional to the translation carry multiple teeth here.

• Global screw: Rolling worm gear Find the power transfer in conventional worm gear sliding takes place, this is solved by rolling in the global roll spindle. This offers significant advantages: a high level of efficiency, zero backlash and a barely existing heat generation.

Materials and lubrication

The production of worm gears places increased demands on the production process in terms of manufacturing accuracy and surface finish to reduce friction. For the same reason the screw and worm wheel are made of different materials; while the larger exposed to wear, for example, the screw is made of hardened steel, the worm wheel is often soft and is made of brass, bronze or self-lubricating plastic.

Exposed worm gears are lubricated with grease in sealed enclosures also oil can be used for lubrication. For example, synthetic oils are used on polyglycol basis for lubrication. For special brass or aluminum bronze PG oils do not come into question, but here can only be lubricated with mineral oils. The oil reservoir is to be addressed by the housing design and the intended heat dissipation ( splash lubrication / oil circulation).

Since worm gear in continuous operation can be heated by the low efficiency at high ratios, it is a further object of the lubricant, carry away the heat as in other gears. Can he easily dispose to the ribbed housing for this purpose or is this even blown by a fan, the oil reservoir may be less.

Requirements for the construction

As the screw is helical gear teeth, but firmly supported, you experience high axial forces. Depending on the operating, whether in only one direction of rotation or in both, the respective sides are sufficiently support for example by thrust bearings. For the worm, the conditions for helical spur gears apply analogously.

Furthermore, it is noted that the screw is also claimed to bending and storage must be performed accordingly elastically or angular balancing. A correct tooth engagement in the resulting by bending the center distance is only guaranteed when using screw involute. For hollow flank worm a malicious edge support can adjust by the deflection.

Manufacturer (selection)

• Benedict Ketterer Söhne ( DE)
• Bonfiglioli (IT)
• Danfoss Bauer ( DK )
• Flender ( DE)
• Gear Nord ( DE)
• Güdel (CH)
• Henschel Antriebstechnik ( DE)
• Lenze ( DE)
• Liebherr Verzahntechnik ( DE)
• SEW Euro Drive ( DE)
• Stöber Antriebstechnik ( DE)
• Wittenstein ( DE)