Open or semi-enclosed gear drives, also known as heavy-duty gear drives or girth gears, have been a common method of power transmission since the beginning of the Industrial Revolution. Generally, open gear drives are the most economical type of gear drive alternative for use in applications where high load-carrying capacity and long service life under severe shock load conditions are required. It is these characteristics in addition to flexibility in the machine's design that have made open gear drives the most common type of drive used for ball mills and grinding mills, kilns, dryers, draglines and shovels. There are two types of open gear drives; Type 1 utilizes a rack whereas Type 2 utilizes a gear or series of gears.

Defining the Application

Type 1 Open Gear Drives

Type 1 open gear drives consist of an actuator and a rack system used to transmit power. This type of open gearing is primarily used on the cable hoist drums, swing motion drives, mechanical boom lifts, shuttle transfer units and in the hoist and drag drives of mining shovels, draglines and excavators. These open gears can be bidirectional in motion, and in most applications the tooth geometry, surface finish, pitch line and intermittent loads cause them to operate in thin lubrication film or boundary lubrication condition. Type 1 gears are typically spur-type gears.

The primary functions of a lubricant used on a Type 1 open gear drive is to act as a friction-reducing interface between the meshing gear tooth surfaces and as a cushion against shock loads. If the open gear lubricant is able to provide these characteristics and contaminants and wear debris do not exceed moderate levels, the open gear lubricant can significantly increase the life of the open gearing.

Type 2 Open Gear Drives

Type 2 open gear drives typically consist of an actuator, pinion and a gear or a series of connecting gears used to transmit continuous loads. They are normally used to power stationary or semi-stationary equipment such as kilns, grinding mills, rotary furnaces, dryers, debarkers, rubber mills, paper mills and finishing mills.

Type 2 open gear drives typically operate at or near their design limits and are often exposed to abrasive contaminants and wear debris. They usually consist of single helical and double helical-type gears.

Open Gear Lubricant Requirements and Industry Standards

Open gearing applications, particularly those associated with ball mill and finishing mill applications, are considered some of the most difficult applications a lubricant can encounter. Generally, these types of open gears operate at low pitch line velocities and/or under heavy loads. However, more recent designs on equipment require open gearing to transmit increasingly higher loads. Because of these considerations, an open gear lubricant must possess the following characteristics and properties:

1. Tackiness (adhesive/cohesive properties) - excellent adhesion to the gears
2. Resistance to water washout and spray-off
3. Load-carrying capability to protect against friction and wear
4. Protection of the gears against wear and corrosion
5. Cushioning ability (vibration reduction)
6. Sprayability and/or ease of dispensability
7. Alleviation of housekeeping and maintenance problems

• Resistance to fling-off
• No buildup in the roots of the gear teeth
• Drainable for ease of removal from guards

Several industry-wide specifications and standards have been developed for open gear lubricants by the American Gear Manufacturers Association (AGMA) and different original equipment manufacturers (OEM). The most widely used is AGMA 251.02 (which has been incorporated into AGMA 9005-D94 and AGMA 9005-E02) and U.S. Steel 226 and 236. The requirements are detailed in Table 1.¹

The AGMA 9005-D94 and AGMA 9005-E02 stipulate the use of residual compounds 14R and 15R for open gearing when the open gear lubricant is applied by intermittent application methods, where the pitchline velocity does not exceed 7.5 meters per second (1,500 feet/minute). The stated viscosities for 14R and 15R are 428.5 to 857.0 and 857.0 to 1,714 cSt at 100^oC respectively.

In addition to the AGMA specifications, different OEMs of mining machinery and open gearing units have their own benchmark tests and specifications. Falk specifies that an open gear lubricant must have a minimum viscosity of 857 cSt at 100^oC, while Sevedala specifies the open gear lubricants have a minimum viscosity of 150 cSt at 100^oC. The remaining OEM requirements for mining machinery are outlined in Table 2.¹

Types of Open Gear Lubricants

Currently, the lubricants used by both Type 1 and Type 2 open gears consist of the following:

1. Asphaltic type (also referred to as residual compounds)
2. Semifluid greases (also know as paste type)
3. Semifluid grease cutbacks
4. Gel/polymer-thickened types
5. High-viscosity synthetics

Each of these open gear lubricants possesses performance and compositional characteristics that define their capabilities.