Timber and Its Use in Commercial Truck Construction – Cập nhật kiến thức mới nhất năm 2023
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Many types of timber are used in commercial truck bodywork today, and it is used in a variety of different forms ranging from sections for framing, chipboard for partitions, to faced plywood for decorative panels. Manufactured timber panels are also used in conjunction with other materials such as aluminum and plastics to produce panels for special purposes which include heat and sound insulation and to improve resistance to general wear and the weather.
Some timbers are more suitable for vehicle bodywork than others because of their superior strength, resistance to wear, decay and fire, or appearance.
Although timber is used in many forms, it is all obtained from either soft wood trees or temperate or tropical hardwoods.
There are a large number of softwood trees, which include many varieties of pine, fur, and spruce which make up about 90 per cent of all timbers used generally in the U.S.A and Europe. These trees are normally found in the northern hemisphere; most of them are cone bearing and evergreen, but there are some exceptions. In fact some types of softwoods are harder than hardwoods.
There is a good supply of softwoods and since less time is required to mature and they are easier to work they are generally cheaper than hardwoods. Softwoods are uses for the sides and floors of mineral carrying vehicles, since it is cheaper and easier to replace. It is also used for manufactured panels for other commercial vehicle bodies.
These are obtained from broad leaf trees, and there are over 2000 varieties including such well known types as oak, ash, teak and mahogany. All basal wood, which is much softer than softwoods, is in fact a hardwood, since it is the structure that decides the group not the texture.
Because the hardwoods take much longer to mature, are more difficult to obtain and work, and take much longer to season they are considerably more expensive. However, they are generally much stronger, more durable and have far more uses than softwoods.
Hardwoods are divided into two groups:-
1. Tropical. These are from trees found in Central Africa, India and South America and include the many different types of teak, mahogany and ebony.
2. Temperate. The varieties found in this group are oak, walnut, and ash are some of the temperate hardwoods found in U.S.A, Europe, Japan and Australia.
Hardwoods are used for framing and in some forms of veneer on panels for interior finishing. Also hardwoods such as ash are very suitable for curved framing members.
Plywood, block-board, chipboard, MDF and hardboard panels are often used instead of solid timber panels, and have many advantages. They can be obtained in larger sizes, are often stronger and are more stable, which means labor costs can be reduced when compared with other methods of covering large areas with equivalent tongue and groove jointed timber.
Plywood is made by gluing layers of veneer together at right angles to each other. This prevents splitting and greatly increases the strength of the panel. Curved members can also be produced by laying the veneers on a suitable mold or form before gluing.
Birch, ash, pine and fir are used in the manufacture of plywood but many other varieties are used for the face side of decorative panels. Plywood is obtainable in several thicknesses and sizes from which vehicle body floors can be made in one piece.
There are a number of manufactured boards making use of solid timber core stock. One of these is block board and this consists of softwood glued together and faced with veneers of timber such as birch and mahogany. Large panels up to about 2 inches in thickness are produced and used in flooring and partitions.
Chipboard is made from graded wood chips which are bonded together under pressure with synthetic resins and adhesives to form large strong panels 0.5 inch to 1 inch thick. These can be faced with many different materials to give painted, plastic or veneered surfaces. It has replaced solid timber for many purposes and because of the sizes manufactured it is a useful vehicle building material.
Medium Density Fiberboard (MDF).
MDF is manufactured by bonding wooden fibers together using glue, heat and pressure. It is a very adaptable material, but can only be used for interior bodywork. It must be sealed / painted as it gradually emits urea formaldehyde which is a known carcinogen and eye, throat and lung irritant. Proper safety procedures should always be used when working with this material in any way.
This is another useful material which, because of its smaller thicknesses is very suitable for interior paneling. It is made from compressed timber fibers and usually has a smooth side with the reverse side having a rough or mesh finish.
Identification of timbers.
Since many types of timber are used in vehicle body work, it is possible to identify the more common ones such as oak and teak. Most types have distinct features and properties, and when samples are carefully examined it is often possible to identify a particular type. The following features should be considered:-
1. The general appearance.
- i) Color – red, brown, yellow, white.
- ii) Grain – close or open, straight or twisted.
- iii) Texture – hard or soft, rough or smooth.
- iv) Figure – shape of grain
Weighing a sample is not a reliable test since the weight will depend on the moisture content and even the weights of samples may vary.
3. Use a hand lens.
Examination of the surface and end grain may reveal distinctive features.
4. Use a microscope.
The use of a microscope will enable a much closer examination of a thin section.
5. Dissolving a sample.
This will allow a microscopic examination of the shape and size of cells and fibers.
It should be remembered that features and color may be affected by the origin, rate of growth, and treatment since felling and many other factors.
One of the main disadvantages with timber is that defects may be present or may develop later. Some of these can be treated satisfactorily but others may require the component to be replaced. Some defects can be avoided completely by careful felling, conversion and seasoning, but a knowledge of the defects will enable you to make the best possible use of the timber that may be somewhat inferior.
These are not always a serious defect since in many cases timber is covered with paint, panels or other disguises and in timbers such as knotty pine they are treated so as to improve the appearance. On the other hand knots should not be present in framing timbers, some decorative panels or other situations where strength or appearance may be affected. Knots are nearly always present and timber is often graded by the distribution and number of knots present. Some timbers have more than others, so careful selection is usually worthwhile.
Splits and shakes.
Shakes appear in various forms and are not always apparent until the tree is felled and sawn into logs. Heart and cup shakes may not present much of a problem but other types such as the ring and star shakes can result in valuable timber being unusable. During the drying out or seasoning the moisture leaves the timber more quickly from the ends than the sides. If this is too rapid other splits may occur.
Many types of timber are affected by various insects, and chemical treatment may be effective. The alternative is to remove and burn the infected part and fit a new piece.
Timber, which is stored in conditions which are damp or lack ventilation will lose its strength and become soft and spongy. If this happens timbers should be destroyed.
This is a common defect and is caused by uneven shrinkage during seasoning. Softwoods are more prone to this, since softwoods contain more moisture than hardwoods. Warping can be reduced by sawing the logs radially instead of tangently. If you examine the end grain of planks it is possible to decide which way they have been cut from the log.
Causes of timber defects.
- i) Storm damage.
- ii) Exposed growing conditions.
- iii) Inexperienced felling.
- iv) Incorrect conversion.
- v) Poor seasoning.
- vi) Bad storage conditions.
- vii) Careless selection and use.
Living trees may have a moisture content of between 50% and 100% and since this is too high the trees, when felled have to be seasoned or dried until the moisture content is reduced. Timber for vehicle body work should have a moisture content of 12 %- 15%. If the moisture content is too high the timber will dry out further and shrink after use. If it is too low the timber will absorb moisture from the atmosphere and possibly swell. To find the moisture content a sample of timber is weighed in its normal condition and then weighed again after it has been carefully dried out. The moisture content is then calculated as follows:-
The moisture Content of the timber (M.C. %) is calculated by taking the wet weight, subtracting the dry weight, and dividing the answer by the dry weight and multiplying by one hundred.
The method of drying out a piece of timber to obtain its moisture content is unnecessary in practice, because electrical instruments are used to give instant readings There are a number of reasons why timber should be properly treated (seasoned):-
- I) It is much stronger than unseasoned timber.
- II) There is less risk of decay and attack by insects.
- III) Painting, staining and other finishing processes can be carried out successfully on seasoned timber.
We should remember that the cells in a piece of timber are like wooden buckets. Large amounts of water in the cavities and the walls, so it is essential for timber to be properly seasoned. After the trees have been felled, the logs are taken to the saw mill where they are converted or sawn into planks or other sections. Since this exposes a greater surface area to the atmosphere seasoning time is reduced. The method used to saw or convert the logs will also affect the shrinkage, warping or other timber defects discussed previously.
The sawn timber is piled in stacks with sticks or wooden strips separating each layer. This allows the air to circulate freely around each piece. The top of the stack should be protected from the sun and the rain, and since moisture will dry out more quickly from the ends of the planks, these are sometimes protected to prevent end splits. The air drying method is cheap and often leads to better quality timber, but it is a slow method, taking up to several years. The timber is liable to staining and insect attack and the moisture content is very rarely less than the surrounding area.
This is a much quicker method, which results in timber with a more closely controlled moisture content. The timber is placed on trolleys and put in a kiln where the temperature and humidity are controlled. Air circulation may be by natural draft or forced by electric fans. Artificially seasoned timbers may be more brittle and other defects may occur, but the reduced time required is big advantage. Some timbers are air dried for a period before being kiln dried.
Seasoning timber processes may take several years and the timber may be stored for a further period before it is used. In order to keep it in good condition certain precautions must be observed:-
- i) To prevent staining the timber should be protected from chemicals and other foreign matter, e.g. soot.
- ii) Excessive rain and heat should be avoided and if stored indoors sufficient ventilation should be provided.
- iii) It should be stacked in a proper manner to prevent boards becoming twisted or warped. The supporting blocks should be directly under each other otherwise the timber will begin to bend.
Wood preservatives suitable for use prior to painting.
This type of preservative is usually of a low viscosity and can normally be used prior to the application of coach or decorative enamels and their painting process. It can be applied by brush, spray or dipping and is normally allowed overnight to dry before application of the above mentioned paint systems. This promotes good adhesion and helps to reduce the problems of timber disease and rot.
Before using this type of product it is essential to make sure that it does not contain any waxes or silicone additives.
Priming of timbers.
Before the priming of any timber it is important to check that the timber is clean, dry and free from any oil residues. The timber should be thoroughly flatted with a suitable grade of glass paper and degreased with a ‘lint-free’ cloth which has been dampened with white spirits or a suitable solvent.
Priming should then be carried out using a suitable primer. It is essential when priming that all areas are adequately primed. This includes timber ends and tongue and grooves, where areas are to be covered with metal fitments or body sections. The reason for this is to prevent the ingress of moisture which would result in paint flaking.
Timbers containing knots should be treated in the following manner, prior to priming:-
I) The knots should be burnt with a blowtorch to extract surplus resin.
II) The timber should be thoroughly rubbed down and degreased as above.
III) Each knot should be treated with one or two coats of shellac, and allowed sufficient time to dry.
IV) Prime the timber as above.
These are non-pigmented finishes. Among those available are wax polishes, sealer coats, long and short oil varnishes, alkyd varnishes, yacht varnishes, single pack polyurethanes and teak oil. All these are suitable for brush application. For spray application only, use the following – cellulose lacquers, two pack polyurethanes, two pack catalyzed lacquers and single and two pack epoxies. The requirements of a wood finish are:-
- i) Color flexibility.
- ii) Build.
- iii) Life expectancies.
- iv) Acid or alkali resistance.
Sealers and wax polishes.
Sealers – These are cellulose based.
Waxes – These are petroleum based. One example is chilled wax which is fairly quick drying, gives little discoloration to the wood, and produces a good average finish.
Waxes – Silicone based. These waxes are very quick drying and produce a hard waterproof finish. The wood needs to be sealed before use, and it tends to give it a cloudy appearance.
Waxes – Beeswax. This is a natural product from honeycomb which has been rendered down with white spirits. It can be applied by brush or rag and produces a good finish that can be re-polished.
Long oil varnishes.
These contain more oil than resin in its formulation. The resin used is usually an alkyd or fossil resin. Long oil varnishes are flexible, due to the amount of oil and have good durability, making it ideal for exterior usage. It also has a fairly clear finish. Polyurethanes are used for wood which is left in exposed conditions, but tends to be rather soft for interior use. The life expectancy is two to two and a half years, and a four coat system is recommended, i.e. apply one very thin (diluted) coat followed by three full coats.
Short oil Varnishes.
These varnishes are quick drying but tend to be rather brittle. They dry to a hard finish and are more suitable for interior use, as they do not have the flexibility for exterior use.
These are clear quick drying varnishes which are suitable for indoor and outdoor use. They are popular as a varnish for vehicle bodies, and are also applied as a four-coat system.
This oil is easy to apply, but liberal coatings are required to restore the natural color of the timber.
This is based on shellac and methylated spirits. It is quick drying and causes the timber to darken. It is applied using a pad, made of a packed piece of wool or gauze, covered by a piece of cotton cloth. It is however, brittle and easily damaged by water and white spirits. It is mainly used in antique furniture.
These lacquers come in two types, the pre-catalyzed and the nitro-urethanes. These contain amino and polyurethane resins respectively.
There is no doubt of the advantages of using timber in commercial truck bodywork today. Certainly other materials have and will come along to try and replace it, but for its versatility and availability it is still unrivalled and will be for a long time to come.
Knowing your timbers and how to look after them will ensure the maximum life expectancy from your truck or commercial vehicle.
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