Hood lifting methods

Hoods and hoods

There are a number of ways of lifting the hood of a top-hat kiln, including hand or electric winch, chain block or electric hoist, hydraulic ram, car jack.

Small hoods can be lifted directly, whilst larger and heavier ones can use rope and pulley multiplier systems.

When considering a hood lift system, there is one safety rule which should always be observed;

“Never work under a raised hood unless there are safety catches to prevent it falling.”

Setting safety catches takes such a short time, yet can eliminate the risk of injury to the operator or damage to equipment.

A properly designed and built top-hat kiln should last up to 20 years or so, albeit with some tender attention during its lifetime. During that time many studios will move at least once, so anchoring a hood lift system to the building structure can drastically restrict ones future options.

Much better to make the kiln and lift system self contained, so all that is needed of the new locale is a flat floor and a power outlet.

The top-hat kilns illustrated throughout this site are all self contained and fall into three groups. There are the;

• Small models (Riley Glass Kilns GS and FS models) with the hood raised by jack sufficiently high to clear any load which may be placed on the hearth. The hearth may roll out, but this is for convenience and not essential to kiln operation.
• High Lift (HL) models where both hearth and lift frame are fixed and the hood is raised sufficiently high that the operator can work beneath it. 
• Rolling Hood (RH) models where the hood and lift frame is rolled away clear of the hearth, either simply to clear the hearth for work or to put the hood over a second hearth to speed up production. Here, the hood need be lifted only as high as is necessary to clear the load on the hearth.

The lift methods and materials are described below in terms of their application in the above situations.

Lift mechanisms

These fall into three main groups;

• Hydraulic jacks or rams
• Hand winches
• Electric drives

Hydraulic jacks or rams

Fig 89-01

A simple system such as that shown in Fig 1 uses a car jack and a system of pulleys to multiply the ram travel of 115mm or so to the hood lift of over 300mm.  

This lift method is simple, low cost and gives precise control of hood position. This latter is important when rapid cooling.

Another system considered would have air rams or oil hydraulic cylinders at each corner of larger hoods, something similar to the ‘jack-up’ camping consoles for mounting on small tray trucks. These were rejected for a number of reasons, including the complexity of maintaining the hood in a level attitude as the equipment aged.

Hand winches

There are two main type of hand winch available;

• Ratchet winch
• Brake winch

The ratchet winch is the type used for pulling small boats onto boat trailers and for similar operations. It has a pawl which engages with a toothed wheel when winding in the positive direction (lifting), but this must be released to wind in the opposite direction (lowering). To start lowering, the full load must be taken by the operator with only one hand, as the other hand is needed to release the ratchet pawl.

There have been many instances of broken or damaged wrists when control has been lost at this point, both with kilns and with boats which have gone into the water somewhat faster than planned. A rapidly spinning handle can do lots of damage. Fortunately, this type of winch is now rarely used on kilns.

Fig 89-03

Of the brake winches available in Australia there are two main gear ratios, 5:1 or 10:1.
Fig 3 above shows a 5:1 ratio brake winch on a custom Riley FS-1 with a deeper than normal hood which could not be lifted sufficiently high by the jack lift system normal for that model.

When lifting, the brake winch operates somewhat like the ratchet winch, except that it is stopped from running back by a brake disc and permanently engaged brake pads. To lower, one simply has to overcome the friction of the brake. No pawl to release, no risk of damaged wrists; although lowering usually requires more effort than raising.

For very large kilns, lifting by brake winch is by far the cheapest method.

Electric drives

These can consist either of;

• An overhead electric hoist running on a girder and trolley, or
• An electric motor and reduction gearbox driving some form of wire rope take-up system.

Overhead hoists will usually have a single anchoring point onto the hood and be arranged so that the hood can be moved off to one side along the girder and clear of the hearth.  Obviously it cannot be a simple lift alone, as one cannot work on the hearth with the hood hanging directly above. There will be a trailing power cable which can sometimes be a nuisance.

Fig 89-04

More self contained is a reversible electric motor driven gearbox and shaft with take-up spools, and with wire ropes going to four lift points on the hood.

Hood is raised and lowered by simply pressing the appropriate button, but there is a complex set of components in the control box to ensure safe operation.

In addition, limit switches must be accurately positioned to limit the upward hood  movement; as well as to prevent unwinding of the rope from the spools when the hood is seated on the hearth. These additional components can add many hundreds of dollars over and above the price of a gearmotor unit to the cost of an electric lift system

Flexible wire ropes

To be able to pass around pulleys of reasonable size, wire ropes must be of the type termed ‘flexible’. They are made up of multiple strands, each strand of which is in turn made up of multiple strands.

A typical rope may have seven strands, with each strand made up of 19 strands; known as 7 x 19. Another may be 6 x 19. In this case the seventh or centre strand will be hemp or some other filler.

For small wire ropes, the size is denoted by the diameter.

There are two types of wire which can be used to make these ropes;

• Stainless steel, usually AISI 304 grade and
• Galvanised high tensile steel.

Of these, stainless steel is much more prone to work hardening than high tensile steel and should be avoided where possible.

Work hardening is caused by continuous flexing which causes the strands to become hard brittle and prone to snapping. Flexing occurs in any situation where the rope is run round pulleys and continuously bent and straightened.

When it occurs it is evidenced by tiny wire ends sticking out like fine needle points from the rope in an area where continuous flexing occurs.

Its effect can be minimised by using pulleys or spools of large diameter. 30 times the rope diameter is recommended as a minimum to avoid it occurring. Often this is difficult to achieve.Fortunately, it can be reduced to about half to 2/3 that ratio if the need to replace the ropes at some time is accepted as part of ongoing maintenance.

On Riley kilns, 2mm SS wire rope is used on GS kilns as galvanised HT steel wire is not available in such a small diameter. Galvanised HT wire rope is used on all other units.

Wire rope loads

Tables are available setting out the recommended maximum load for each wire rope size and strand combination. 

These numbers must be de-rated to allow for the inertia of the load when the strain is first taken up. Consider an electric motor which instantly goes to full speed and commences winding in the rope. The load doesn’t want to move so the load on the cable increases way past the actual mass of the hood itself. As soon as the hood is moving at lift speed the load will decrease. A de-rating factor of about 5:1 is used for these situations; the hood is assumed to have a mass five times its actual mass.

A similar factor should be used with jack lift systems because, with the hood hanging stationary on the rope, each pump of the jack is another starting load on the rope.

A slightly lower margin could be used with hand winch lifting, as we humans will tend to overcome the initial inertia a bit less strenuously than will an electric motor.

Wire rope components

Fig 89-05 shows the components which can be combined to make up a rope lift system.

All these can be obtained from specialist rigging suppliers or from some engineering hardware outlets.  

Each item is suited to the rope diameter and, in the case of the swage, also to the rope type – different materials are used for swaging SS rope than is used for galvanised steel. This is because of the risk of galvanic corrosion should moisture accumulate inside the swage.

Swaging

Involves the deforming and compressing of the swage tightly around the two rope pieces. The swage is held in a die of the appropriate diameter and compressed, usually in some type of hydraulic press but can sometimes be done with hammer blows.

Pulleys

Fig 89.06

To prevent damage to the wire rope the groove in which it runs should be matched to its diameter, with the groove for smaller ropes about 1mm larger than the rope. The running of small diameter rope in large radius grooves should be avoided; and this increases the difficulty of sourcing pulleys.

Fig 6 shows some of the range of pulleys with sealed ball bearings which have been custom made to meet the need of various Riley Glass Kilns designs.

The ‘dumbbell’ shaped SS pieces in fig 6 are bent to become pulley blocks for the lift systems in FS (3mm galvanised wire rope) and GS (2mm SS wire rope) kilns. In both cases two lift cables are used for safety reasons, so the pulleys used in pulley blocks are double grooved. One rope is able to carry the load but the second length is made slightly longer as 'back-up' should the first one fail.

Pulley & rope multiplier systems

Sometimes the load may be too great to be handled by even a 10:1 ratio winch, and further reduction is necessary. This can be done using rope and pulley looping, where each loop reduces the load by one half; albeit at the price of twice as many turns of the winch.

Fig 89.07

Fig 7 shows such a system used on a 4500mmm (14 1/2ft) long kiln with 10:1 reduction on the brake winch and two 2:1 reductions on rope/pulleys; giving a 40:1 lift ratio. Note the difference in the distance allowed for travel of the right hand block compared with that on the left. This is because each succeeding doubling requires double the travel distance.

Safety devices.

These should be matched to the level of hazard existing and protection needed.

Fig 89-08

In the case of the Rolling Hood kiln shown, failure of the winch brake could allow the hood to drop, either misaligned onto the hearth or clear of the hearth. The safety catch marked, if swung into the path of the winch handle, will stop it from turning clockwise and allowing the hood to fall. The winch shown is a 10 to 1 ratio unit; with the smaller 5 to 1 ratio model the handle is wound in the opposite direction for raising, so the position of the safety catch would need to be changed. Dropping of the hood due to failure of any other lift component is more predictable and eliminated by regular inspection.

Fig 89-09

In the case of High Lift kilns, where people will work under the hood, a greater level of safety is required. Fig 9 shows one of the safety catches used. These swing in at each end so that they are directly under the bottom edge of the hood and provide positive support so that it cannot fall under any circumstance.