A steel ship is constructed from an assembly of interconnected steel frames to which the steel plates are riveted or welded. This applies to the hull, the decks, the superstructure and the roofs of, for example, the deckhouse and the sea cabin. Many of these frames are pre-bent to a greater or lesser degree. That is required in order to be able to achieve the required curvatures in the hull, the decks and the roofs. By applying a curvature to surfaces the construction becomes stronger, and the decks and roofs also drain water and the hull is given a shape that can cut through water effectively. A frame bending machine is used to create a curvature in the frames.
The correct curvature for a frame is marked out on a frame bending floor (image 5). This is a large, flat wooden floor onto which all of the curvatures of the frames are drawn out in pencil (NB The floor in the photograph is not sufficiently flat!). The frame bending floor is used to check during the bending process whether the frame has reached the required curvature. A frame is made from a section of steel or iron, such as a piece of angle iron or a T-section. You proceed as follows:
- Take a piece of steel of the required section (for example angle iron or, as shown in the photographs, T-section) and more than sufficient length. Make sure that there is sufficient extra length because extra length is required for clamping the frame into the frame bending machine so that you are also able to bend the ends. However, extra length is also required because more material is required for the curvature of the steel section. The length of the bend between two points is, after all, longer than a straight line between the same two points. The greater the bend (image 3), the more extra length you require. You can measure the extra length required on the frame bending floor. To calculate the correct curvature there are also formulae available (image 1) which have been derived from Pythagoras’s theorem.
- Place a length of section onto the frame bending floor and use a crayon to draw a line in the centre of the length of the section (image 4). Now draw successive lines, equally spaced outwards towards the ends. These are the bending points at which you will bend the section in the frame bending machine. For a more pronounced curvature you use more bending points than for a lesser curve and these ensure that you obtain a flowing curve without visible kinks.
- Now place the T-section (a heavy section should be lifted by two people) onto the frame bending machine (image 8). Make sure that the section is positioned completely flat and horizontal on the working bed of the frame bending machine.
- Position the section against the two steel pressure blocks (image 12). There are various shapes of steel pressure blocks to adapt the frame bending machine for bending various types of steel sections.
- If necessary you can support the section using blocks of timber. The section must lie in a stable position and must not be able to topple over.
- Slide the section so that the centre of the length of the section is positioned in the centre of the two steel blocks (image 11). You actually start bending from the centre of the section outwards in order to form the required curvature in the frame.
- Now turn the spoke wheel (image 9). For heavier frames it may take three or even four people to operate the frame bending machine: two people hold the section, one at each end (image 19), and one or two people turn the spoke wheel. For light-weight sections you can turn the wheel by hand; for heavier sections you have to apply all of your weight to the spokes in order to transfer sufficient force to the frame bending machine.
- By turning the spoke wheel the reciprocating sliding block in the frame bending machine is pushed forward, towards the section. As you continue to turn the wheel the block bends the section (image 16). Start with care and do not push too far: you can always bend a curve further. A frame that has been bent too far can always be turned over and bent back, however, bending back does weaken the material.
- If you now turn the wheel in the opposite direction the block will move away again from the section. The curvature in the section will spring back very slightly.
- Now slide the section slightly further, until the next chalk mark is positioned in the centre between the two pressure blocks. A part of the curvature is also applied to this bending point by operating the spoke wheel of the frame bending machine.
- Continue repeating this operation until you have formed a curve over the entire length of the section. When turning the spoke wheel make sure that you turn it the same amount at every bending point so that the section is always bent by the same amount at each bending point. In this way you will create an even curve in the frame.
- Now remove the section from the frame bending machine (image 15) and place it on the frame bending floor in order to check the curvature (image 18). Place the frame back on the frame bending machine and repeat this process if the section needs further bending (image 17).
- Make sure that you always bend small areas each time and do not be tempted to create a tight curvature in one go. By repeatedly bending small areas further you will achieve a nice, even curvature without kinks.
Formula for calculating 'curvature'
The position of the bend
Mark the centre point
Frame bending floor
Place the frame onto the frame bending machine
Turn the spoke wheel
Start bending in the centre
Pressure blocks, sliding block and timber support blocks
Remove the frame from the frame bending machine
Block bends the section
Repeat bending and checking
Checking for the correct curvature
Working as a team