Clutch Designs for Equipment Controls Optimization

Farmers have long been aware that they are losing money by double planting on end rows, point rows, around terraces and by planting unsuitable areas such as waterways. With seed corn at $200 a bag, the amount of money wasted in dropping seed where it is not needed is substantial. Another concern is double planted areas often run out of moisture and nutrients, resulting in yield loss.

Some farm equipment OEMs have addressed this concern by providing the operator with the ability to manually turn off sections of the planter. This approach provides significant benefits, however, it places an additional burden on the operator, and controlling planting at the section rather than the individual row level still wastes a considerable amount of seeds.

Adoption of automated control
Over the past few years, there has been rapid adoption of automated control for sprayers applying fertilizer and chemicals. A number of companies offer automated control of the three to five sections into which most sprayers are divided, and a few companies have introduced control at the individual nozzle level.

Now, OEMs are working on upgrading planters to automatically control individual row units while planting. The basic idea is to automatically turn rows off as they enter covered areas, or areas that are marked not to plant on the coverage map. This requires the GPS system be integrated with a control system that compares the planter's location to the coverage map, and sends a signal to disengage a clutch integrated with the seed meter drives at appropriate times.

Normally the seed meters for the individual rows are driven by flexible cables through a gearhead. Controlling the seed meters requires that, for each row, a clutch be added that can engage or disengage the power from that row's seed meter.

From a mechanical standpoint, this application requires the seed meters be turned on and off with very little delay in order to deliver seeds exactly where they are needed without overplanting. From a standpoint of reducing the development, manufacturing and operating costs, the drive system should be as small, light and power-efficient as possible. In addition, the rigors of operating in the farming environment of course require a very rugged design and one that is protected from the elements.

Up to now, the normal approach has been to retrofit pneumatically-powered clutches to existing seed meter gearheads so that individual rows can be turned on and off. One weakness of this approach is that a pneumatic power system needs to be added to the planter. This adds cost and weight, as well as another type of power system that needs to be maintained and supported over the life of the equipment. Pneumatic systems also consume power whether or not they are being used. The use of a separate clutch with its own enclosure also increases the size and weight of the installation. In many cases, extensive modifications will have to be made to install this type of device such as cutting and re-welding the frame.

Wrap Spring Clutches
Thomson is a supplier of wrap spring clutches (Deltran brand) and also a major supplier of gearheads (Micron brand). The company recently took advantage of its capabilities in these two areas to integrate a wrap spring clutch and gearhead into a single enclosure to provide a complete row control solution for planter OEMs.

The basic wrap spring clutch consists of three elements: an input hub, an output hub and a spring whose inside diameter is slightly smaller than the outside diameter of the two hubs. In its basic form, the wrap spring clutch operates as an overrunning clutch. When the input hub is rotated, the spring wraps down to engage the two hubs. If the input is stopped or reversed, the spring unwraps to release the output hub, allowing the load to overrun. The greater the force of rotation, the more tightly the spring grips the hubs. The torque capacity of the spring clutch is a direct function of the diameter of the hub and the tensile strength of the spring.

The addition of a stop collar and a control tang on the spring lets the output hub start and stop while the input hub spins. Stop collars come with one or more stops, up to 24/rev or 15 degrees between stops. The control tang anchors to the stop collar, which surrounds the spring and hubs. An external mechanism engages a lug on the stop collar OD. Halting the collar unwraps the spring and releases the output hub. The arrangement applies no braking to the output hub.

Wrap spring clutches provide several advantages in planter row control applications. For example, wrap spring clutches are considerably lighter and smaller, and require only 1/10 to 1/5 of the power of a friction clutch for a given amount of torque. Wrap spring clutches also provide faster response than friction clutches by rapidly syncing up loads with the drive motor within a predictable time or rotation angle. Unlike friction

Magnetically Actuated Clutches A magnetically actuated clutch is a variation of the wrap spring clutch consisting of an integral coil and the same three basic elements-an input hub, an output hub and a drive spring-but whose inside spring diameter is slightly larger than the outside diameter of the shaft hub. This is a unidirectional random start/stop type of wrap spring clutch that does not use the traditional stop collar or an external mechanism to engage or disengage the unit. The driving and disengaging functions of this unit are determined by how long the coil is energized or de-energized. When the coil is not energized the clutch input or output will free wheel in both directions. Magnetically actuated clutches have an integral control collar that couples to the drive spring to engage the clutch when the coil is energized. When the coil is energized, the angular displacement between the drive spring inside diameter and the output hub outside diameter are coupled together transmitting torque to the output shaft. The unit will continue to drive the output shaft until the coil is de-energized. If the coil is re-energized, the unit will then drive the output shaft until the coil is de-energized. This same type of unit can be designed to function as a reverse logic clutch. The reverse logic function allows the clutch to drive without the coil being energized. In this configuration, when the coil is energized the clutch would disengage and stop driving.

clutches that can slip under certain conditions,  wrap-spring clutches won't slip when engaged. The reverse logic approach needed for this application was developed by modifying a standard wrap spring clutch so that it is normally engaged and only disengages when the control tang is activated.

User ROI
The ability to shut off individual rows can generate substantial cost savings. A typical planter working on corn acres plants 12 to 20 seeds per sec per row, representing a cost of about 50 cents per sec for a 12 row planter. Some estimate that the ability to individually control rows based on a coverage map and GPS data can provide savings of over $2,000 compared to a planter without swath control, and just under $1,000 compared to a planter with manual swath control on an approximately 1,400 acre operation. Some estimate that yield improvements are closer to $4,000 in value compared to the planter without swath control, and around $1,500 compared to the planter with manual swath control.

Clint Trenor is global account manager, John Pieri is Deltran product line manager and Mike Leone is senior applications engineer at Thomson Industries Inc.

For more information, visit: www.thomsonlinear.com.