Dearborn Mid-West Company

Overhead / Inverted Power and Free Systems


Overview

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A power and free (P&F) conveyor consists of a continuously-moving powered chain, which when engaged, moves a free (non-powered) load-carrying trolley through the conveyor system. The powered chains are typically drop-forged rivetless - link type (such as X-458 or X-678).

In a P&F conveyor system, force is applied to the free load-carrying trolley via a retractable dog, which is integral to the front free trolley. This retractable dog engages a dog which is integral to the power chain. Power is then transmitted to the free trolley. The retractable dog on the free trolley can be engaged and disengaged through the use of a blade-type stop mechanism, or by the mechanical reaction of the load carrying trolley accumulating on a trailing trolley in the free track.

A typical carrier arrangement is supported by two or more free trolleys using pivoting, load-bearing king pins. The trolley that leads is the front (accumulation) trolley. The rear or trailing trolley is equipped with a ramp or beaver tail that provides the means to begin the accumulation process.

P&F conveyors offer one performance characteristic that sets them apart from the overhead trolley conveyor family, i.e., built-in flexibility. This feature allows them to accomplish the following functions within the same system without impacting simultaenous operations occurring in other areas of the conveyor system:

  • Move product through production areas on specific job centers.
  • Stop in place for load and unload operations.
  • Accumulate loaded and empty carriers for high-density storage.
  • Speed changes as required to maintain a specific production rate or to create buffer zones.

6” OHPF – General Assembly Production Conveyor


Power and Free Conveyor Selection

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The P&F conveyor design offers considerable flexibility in layout. These systems can be designed to move a product in a simple monoplane loop or they can carry products through a variety of elevations and complex layout configurations. They easily accommodate extreme changes in elevation, which can help to maximize use of floor space.

P&F systems offer a wide range of load capacities, sizes, and operating speeds. Speeds range from extremely slow, 1-2 ft (0.3-0.6 m)/min, up to 60 ft (18.3 m)/min and higher for special transfer applications.

4” IPF – Instrument Panel Accumulation Line


Manufacturing and Production Flexibility

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The ability of P&F conveyors to accommodate load-carrying free trolleys on minimal centers provides a great degree of system flexibility. Loaded carriers conveyed on production centers may be completely purged from certain work areas and accumulated in a much smaller space. The carriers can then be allowed to re-enter the production process in sequence, if required.

The accumulation feature permits high-density storage, which can be used for storing empty carriers when not in use, creating product sortation and sequencing banks, or storing product in production surge banks. Sophisticated systems can be created using multiple chains to transfer carriers with product throughout the plant from process to process.

6” IPF – Delivery after Chassis Decking


Reliability and Maintainability

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A P&F conveyor is an inherently simple and reliable method of transporting products. A minimal basic maintenance program will result in many years of trouble-free service. Special attention must be paid to monitor the maintenance needs of the load-carrying free trolleys. These trolleys are vital to the proper functioning of the conveyor system. The free trolley's moving parts must be maintained for proper accumulation to occur. The load and side guide wheels and trolley pivot points all require regular lubrication to prevent premature wear. For ease of maintainability, P&F conveyors are typically equipped with automatic lubrication systems.

6” OHPF – Delivery from Paint Shop


Protecting Product Quality

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Specially designed carriers are used to protect product quality, and by having a system that can be stopped at load and unload areas without affecting other areas of production also enhances product quality. Carriers are transferred and lowered at different speeds to ergonomically adapt to the work process and production personnel.

P&F carriers are generally supported using pivoting load pins from two free trolleys. In situations where more accumulation space is required between the loads, a tow bar and trailing bar can be added. With the use of these additional spacing bars, the carriers can have a free trolley arrangement that encompasses three, four, or more trolleys to suit the needs of a specific application. The carriers themselves can be fixed or rigid construction, or they can have moving parts to adapt to a product. The design of these product-specific carriers protects product quality while also increasing the flexibility of the load, production, and unload functions.

6” IPF – Engine Line Build System


Operator Safety Issues

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Because OHPF conveyors suspend products below or to the side of the power source, workers do not normally come into contact with any of the components of the drive mechanism. The continuously-moving power chain and the load carrying trolleys are elevated away from the normal traffic flow on the work floor; only the product and its carriers are positioned at working height. The products being conveyed through the production area are set at a pre-determined spacing and speed, which maintains adequate clearances between the moving products.

The accumulation of products and empty carriers usually takes place in high-density storage areas located away from traffic flow (To maximize building cube or space availability, this accumulation often takes place overhead). To protect workers from a falling product or debris, safety guarding is typically installed beneath the elevated portions. In addition, drip pans are often installed in certain areas to catch excess lubricant, dirt and dust.

6” OHPF – Engine/Axle Stuff (Lowered)


Equipment Cost Considerations

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An efficient, rugged design, coupled with low manufacturing, installation, and maintenance costs, and a high degree of system layout flexibility, makes the P&F conveyor a cost-effective material handling option.

When evaluating equipment costs for P&F Conveyor systems, flexibility is a top priority benefit. Generally, a P&F conveyor costs more than an equivalent overhead crane solution or overhead monorail conveyor. Ultimately, the correct conveyor selection must be based on an overall cost to benefit analysis.

6” IPF – Inverted Decking to Flat Top Final Line


Types of Power & Free Conveyors

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6” OHPF – Door Delivery


Overhead Power & Free (OHPF)

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In OHPF conveyors, the powered track is above the free track, and the carrier is suspended from the free trolleys by means of load pins. This configuration, with the product suspended below the P&F track, offers excellent product access. This ease of access enables workers to perform many different types of operations, whether the products are moving or stationary.

DMW's OHPF systems are essentially comprised of two standard product lines - 4" OHPF and 6" OHPF.

6” OHPF – Engine/Chassis Stuff


4" OHPF

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This system consists of a powered chain track of 4" (10 cm) structural I-beam, and a free trolley track consisting of two 4" (10-cm) structural channel sections mounted toe-to-toe. The powered chain is a standard drop-forged, rivetless-link 4" (10 cm) pitch chain, supported by trolleys that ride on the lower I-beam flange. The load-carrying free trolleys are captured in the two-channel free track and propelled along as needed by the continuously moving powered chain pusher dog.

The term "444 power-and-free" is derived from the structural and mechanical components of the system: the high-carbon 4" (10 cm) I-beam powered track, the 4"(10 cm) pitch chain, and the 4" (10 cm) high-carbon, dual-channel free trolley track. The industry standard dimensional drop from the top of the I-beam to the centerline of the chain is 7 3/16" (18 cm).

4" OHPF systems are used for medium load applications. The 4" load-carrying free trolleys have a nominal load capacity of approximately 800 lbs. (363 kg). However, because the product is typically carried from a load bar that links two or more trolleys in tandem, the actual load capacity may be as high as 1,600 lbs. (726 kg) for monoplane systems.

4” OHPF – Seat Delivery Conveyor


6"OHPF

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For applications involving heavy product loading and large system layouts, a 6" (15 cm) OHPF conveyor is often the right choice. This heavy-duty system uses a 6" pitch forged power chain supported by trolley wheels which ride on the lower flange of a high-carbon 4" I-beam powered chain track. The load carrying free trolleys are captured in the free track section consisting of two 6" high-carbon structural channels mounted toe- to-toe. The accumulation mechanism and method of operation are identical to the 4" OHPF conveyor.

As was true in the description of the 4" OHPF system, the term for the 6" OHPF - "466 Power and Free" is derived from the track member sizes and the 6" pitch of the powered chain. The industry standard dimensional drop from the top of the I-beam to the centerline of the chain is 7 7/16" (19 cm). The carrying capacity of each free trolley is approximately 3,000 lbs. (1361 kg). However, because the product is typically carried from a load bar that links two or more trolleys in tandem, higher loads up to a maximum of 18,000 lbs. (8,182 kg) can be transported using a 6" OHPF monoplane system.

6” OHPF – Safety Decline Conveyor


Inverted Power & Free (IPF)

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As the name suggests, the IPF conveyor is located below the conveyed product. The product is mounted on a pedestal type carrier allowing virtually 360 degree product accessibility and is ideal for operations involving automation interface. With the conveyor underneath the product, the danger of debris and contamination falling on the product is eliminated. IPF can be on the floor, flush with the floor, or elevated as required. When the conveyor is installed flush with the floor, the conveyor chain, trolley, track, etc., are protected from debris and contamination by means of a reinforced, overlapping, flexible, thermoplastic polymer seal. The seal does not deter the efficient operation of the conveyor.

Seat Stripper


4" IPF

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This system consists of a powered chain track of 4" (10 cm) high carbon structural channel, and a free trolley track consisting of two 4" precision roll formed channel sections mounted toe-to-toe. The powered chain is a standard drop-forged, rivetless-link 4" (10 cm) pitch chain, supported by trolleys that ride on the lower flanges of the 4" power track. The load-carrying free trolleys are captured in the formed channel free track and propelled along as needed by the continuously-moving powered chain pusher dog.

Four inch IPF systems are used for medium load applications. Similar to the 4" OHPF system, the 4" load-carrying free trolleys have a nominal load capacity of approximately 800 lbs. (363 kg). Using tandem trolleys, loads up to 1,600 lbs. (726 kg) can be carried on monoplane systems.

4” IPF – Instrument Panel Build Line


6"IPF

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For inverted applications equivalent to 6"OHPF, a 6" (15 cm) IPF conveyor is often the right choice. This heavy-duty system uses a 6" pitch forged power chain supported by trolley wheels which ride on the lower flanges of high-carbon 4" structural channels. The load carrying free trolleys are captured in the free track section consisting of two 6" structural channel sections mounted toe-to-toe. The accumulation mechanism and method of operation are identical to the 4" IPF conveyor.

Similar to the 6" OHPF system, the load capacity of each 6" IPF free trolley is approximately 3,000 lbs. (1361 kg). Using tandem trolleys, loads up to 9,000 lbs. (3,715 kg) can be carried on monoplane systems.

6” IPF – Chassis Build System


6” IPF – Delivery after Chassis Decking

4” OHPF – Doors Off

6” OHPF – Engine/Axle Stuff (Raised)

4" OHPF - "Doors On"