This article describes three unique transport/ transfer systems used in continuous industrial furnace systems. These charge transfer systems provide unique applicability for specific working conditions and products.

Industrial furnace manufacturers offer a wide variety of transport/ transfer systems for continuous furnaces. The most common of these systems include mesh belt, roller hearth, walking beam, pusher, or car-type mechanisms to drive the workload through a furnace. Each mechanism provides advantages for a particular type of metals thermal process depending on load size and weight, speed, and work capacity or volume required.

Three other transfer mechanisms described here include supported roller hearth, worm screw, and a relatively new, inclined spherical head roller system used for long parts with varying cylindrical or conical sections. All of these systems provide robust solutions for transferring loads through continuous furnaces.

Fig. 2 Schematic section of a supported roller hearth furnace.
SUPPORTED ROLLER HEARTH FURNACES
Supported rollers (Fig. 1) are an important innovation in systems used to transport of materials through industrial furnaces working within a temperature range of 1112 to 2100 F (600 to 1150 C). These rollers are constructed from refractory steel bars and are supported along their entire length by a cast, refractory steel cradle placed directly on the furnace hearth (Fig. 2). Each roller is motorized externally by means of a double universal joint connection that allows the free settling of the roller into the cradle (see Figs. 2 and 3, far right).

Fig. 3 The rollers are supported across entire their length through the furnace. The universal joints from the individual drive motors can be seen in the far right.
Unlike conventional rollers, which are supported only at both ends and subject to creep at high temperatures, continuously supported rollers eliminate the tendency towards hot deformation. The specific load supported on the hearth may vary from 100 to 300 lb/ft2 (500 to 1500 kg/m2), according to the operating temperature.

This system provides other important advantages as follows:

  • the ability to stop a 25,000 lb. (5,507 kg) workload on the rollers in a 1750°F (955 F) furnace with out damage to the furnace;

  • the elimination of water-cooled roller shafts even with operating temperatures up to 2012 F (1100 C);

  • the short pitch between the rollers allows for the transport of many parts without the need for trays;

  • unlike the pusher furnace, which has limited useful lengths, the supported roller furnace can be any reasonable size and can be emptied completely; and

  • the ability to move two-ton loads at temperatures of 1960°F (1070°C) with a motor size as little as 15 Hp.

The aforementioned advantages of the supported roller hearth furnace make the system feasible as an alternative for all high production and high temperature roller hearth and pusher furnaces in all applications.

Fig. 4 General view of a continuous hardening and tempering installation for oil drilling bars of large diameter. The line includes a heating furnace and a tempering furnace using the worm screw transfer system and a horizontal quenching system utilizing laminar water jets.
WORM SCREW TYPE FURNACES
A worm screw transport system (Figs. 4 and 5) is applied in heat treating furnaces for transportation of cylindrical parts such as bars, tubes, or generally large, cylindrical pieces. The worm screws, made of refractory steel and supported along their length, keep parts rolling while moving through the furnace, thus providing rapid, uniform heating. The strength in the translation plus rotation of the charge is reduced when compared to that resulting from a walking beam system.



Fig. 5 Schematic diagram of the worn screw transfer system.
Unlike walking beam furnaces, the worm screw furnace has no openings in the hearth and no water-cooled joints. When present, these features can allow heat loss resulting in inhomogeneous temperatures in the products. A worm screw furnace can reduce the product heat up time drastically as the rotation of the product while moving forward ensures the best heat distribution on its surface, making heat exchange more effective and rapid.

The results obtained in bar hardening worm screw furnaces confirm this fact. A bar of SEAE 4145 steel, with 100 mm diameter, requires a total cycle time of 100 minutes in the furnace operating at a temperature of 1525 to 1545 F (830 to 840 C). A bar with a 250 mm diameter and same steel quality remains in the furnace for 300 minutes at a heat treatment temperature of 1525 F (830 C). These cycle times were obtained by customers whose furnaces have been in operation for over 10 years through optimization of the thermal cycle. The results prove the efficiency of heat transfer to the products in the furnace. The reduction of the holding time in the furnace also ensures less oxidation of the product surface, thus reducing the quantity of scale produced during the quench process.

Fig. 6 A small exit window for the product allows for relatively efficient use of protective atmospheres within these furnaces.
Another aspect not to be neglected is the small size of the furnace openings that are reduced at the load and unload doors (Fig. 6). This feature makes the use of protective atmospheres with these furnaces especially easy.
The most common applications for these furnaces include:

  • hardening of steel bars or gas bottles;
  • solution heat treatment of stainless steel bars;
  • rapid heating of bars to be coiled in helical springs; and
  • rapid heating of brass billets for subsequent extrusion.

The worm screw hearth furnace has very simple construction and requires very little maintenance, which reduces life-cycle costs. Nearly all maintenance is concentrated exclusively on the mechanical systems outside of the furnace. The mechanical parts in the furnace are restricted to just two roller runways to load and unload the work and a transfer worm screw system. The worm screw lifetime can be over 5 years in furnaces treating lighter weight products such as gas bottles or bars of small diameter and length. An annual inspection is typically enough to check the screws and the cradles in which they are housed.

The workload comes into contact with the screw near the thread tip. There is no need for a mechanism to push the work from the load roller runway to the first pitch of the screw, except in special applications with products of large dimension. The screw size and generally all the dimensions of the furnace hearths are suitably designed to avoid any contact between the workload and the worm screw core. Contact could cause serious consequences such as side-slipping of the work in the furnace, or anomalous wear of the screw core resulting in the breakage of the screw.

Fig. 7 Intermediate supports inserted between the worm screws prevent the bar from deflecting under high working temperatures.
The worm screws have limited length (usually not over 9 meters) since the system is subject to high torsion loads. However, furnaces of longer length have been built by coupling two worm screw systems, driven at the opposite ends of the furnace so as to move the workload from the first worm screw system to the following one. This solution has allowed the development of a gas bottle hardening and tempering furnace measuring over 12 meters in length with a capacity of 60 gas bottles/hour.

A worm screw furnace installed in the Czech Republic operates in dry nitrogen protective atmosphere to prevent the oxidation of the products. The furnace design allows the treated products to be of any length. For example, steel bars up to 17 meters long have been manufactured. To move bars of such length and diameter (over 300 mm), no more than 6 worm screws are needed. However, when very long products are treated, intermediate supports are inserted between the worm screws to prevent the bar from deflecting under high working temperatures (see Fig. 7).

Fig. 8 A schematic view of the inclined spherical head roller system employed in heating and tempering furnaces.
INCLINED SPHERICAL HEAD ROLLER SYSTEM
Another innovative transfer system is the inclined spherical head roller system (Fig. 8). This system is utilized for horizontal hardening and tempering of long parts with variable cylindrical or conical sections such as mandrels, special oil drilling bars and cannon barrels.

This type of continuous furnace is provided with a transfer system employing pairs of inclined spherical head rollers that support the workpiece, moving it forward while rotating (or just rotating). The rollers continuously fit the variable shape of the workpiece, which keeps its longitudinal axis steady.

The basic characteristic of this system consists of the rapid and uniform heating of the workpiece while it is kept rotating, and alternately moving forward and/or backward through the furnace during the heating process and also through the continuous quenching of the product. Ring-like water jets simultaneously cool or quench the inner and outer surface of the piece as it is rotating. The treated piece shows exceptional straightness, removing the need for any subsequent straightening process. IH