Crossed Helical Gear Screw Gears

Crossed Helical Gear (Screw Gears)

Crossed helical gears, also sometimes called screw gears, are helical gears used in motion transmission between non-intersecting shafts. The helical gears used in parallel shafts have the same helix angle but in opposite directions. An arbitrary helix angle can be chosen for shaft angles other than the parallel. Assuming no profile shift, the intersecting angle of the shafts is the sum of the respective helix angles. The crossed helical gears have point contacts and have lower efficiency than parallel or perpendicular shaft gears and cannot transmit heavy loads. And crossed helical gears are suitable for light load applications, machine tools, camshaft smell IC engines, etc.

Crossed Helical Gear Description

Crossed Helical Gear Design

1. The geometry of crossed helical gears
2. Crossed helical gear calculation
3. Crossed helical gears with external and internal teeth
4. Collision check
5. Crossed helical gear with rack
6. Fine sizing for crossed helical gear

Crossed Helical Gear Features

While the helix angles must be identical (but with the different hands of the helix) when pairing helical gears, paired crossed helical gears have different helix angles (but with the very hand of the helix). The transmission ratio depends on the ratio of these helix angles.
As the name suggests, the crossed helical gear no longer shows a pure rolling movement during the engagement but a screw motion. Typical for screw motions is the permanent sliding of the flanks. Thus, there are no points on the reference bodies of crossed helical gears to which a pure rolling process can be assigned (i.e., the circumferential speeds of the gears are not identical at any point). The reference bodies of crossed helical gear are no longer “pitch bodies” but so-called rotational hyperboloids! A hyperboloid is obtained by rotating a skew straight line around an axis.
The constant sliding of the flanks usually requires special lubrication of the crossed helical gears (hypoid gear oil); otherwise, increased wear is expected. Due to the screw course of the teeth, the flanks no longer touch each other line-shaped, but the contact is punctiform (exception: worm gears). In addition, the screw tooth path causes strong lateral forces, which must be absorbed constructively by an appropriate bearing.
Therefore, crossed helical gears are designed for moderate torques and speeds, e.g., for drives for machine tools. The use of crossed helical gear also has a disadvantageous effect on transmission efficiency, which is lower due to the sliding processes on the flanks.
The advantage of crossed helical gears, in addition to the already mentioned oblique arrangement of the gear axes, is their low-noise operation. In addition, crossed helical gears can be shifted axially within relatively wide limits without negatively influencing power transmission.

Differences Between Parallel Helical Gear and Crossed Helical Gear

Crossed Helical Gear Applications

Crossed helical gears are working on many lift/elevator and escalator-drive applications due to their compact size and the non-reversibility of the gear. Crossed helical gear is working in presses, rolling mills, conveying engineering, mining industry machines, on rudders. Crossed helical gear has lower power transmission capacity due to point contact. It has a wide area of applications ranging from small equipment to large industrial fields including gear trains. It is rarely working in small-size instruments.

Gear Transmission Maintenance

Proper maintenance is necessary for extending the service life of gears for ordinary working of gear transmissions. The daily maintenance mainly includes the following contents:

1. Installation and running-in. The fixing and positioning of the gears should meet the technical requirements during installation. Using a pair of new gears to run in first, that is, run for more than ten hours to dozens of hours under no-load and step-by-step loading. Then clean the box and replace it with new oil before use.

2. Proper use of gear transmission, start-up, loading, unloading, and gear shifting, should strive to be stable to avoid impact loads, and Party B is causing teeth to be broken and other failures.

3. Frequently check the condition of the lubrication system, such as the amount of lubricating oil, the state of oil supply, the quality of the lubricating oil, etc., to avoid the occurrence of impact loads, and regularly replace and supplement the lubricating oil of specified brand according to the user’s specifications.

4. Monitor the operating status, watch, touch, and hear for abnormal sounds if the cargo box is overheated, and “work with fault” is prohibited. For gear transmission at high speed, heavy loads, or on essential occasions, an automatic detection device can be adopted to ensure the safety and reliability of gear transmission.

5. Install the protective cover. For open gear transmission, the protective cover should be installed to prevent dust, chips, and other debris from entering the tooth surface, accelerating the wear of the tooth surface, and protecting personal safety.