What Is a Plumb Bob?
A plumb bob is a tool used to measure the verticality of building columns and walls.
A spindle-shaped weight, called a plumb bob, is held at the end of a string, which is hung from a plumb bob holder to measure verticality.
Although a level that measures bubbles in a glass container filled with liquid can also be used to measure verticality, the primitive plumb bob, which uses gravity, is a relatively more accurate way to measure verticality.
Plumb bobs that use a laser instead of a string and weight are also available. The laser type has the advantages of not being affected by wind and not requiring rewinding of the string, but it also has the disadvantage that the laser will be tilted if the mounting surface is tilted. For precise measurement, some products are equipped with a correction function.
Uses of Plumb Bobs
Plumb bobs are used not only to measure the perpendicularity of columns and walls but also to confirm that measurement devices are installed parallel and perpendicular to each other, such as when installing a transit, and to raise the ground marker.
There are various types of plumb bobs, depending on the environment in which they are used.
Plumb Bob Installation Procedure and Precautions
- Plumb Bob Installation Method
If the wall to be installed is made of wood, use a push pin (needle). If the wall is concrete, hook the metal hook onto a nail or the like. - Wind Protection
Plumb bobs are vulnerable to wind due to their structure. In recent years, some are equipped with a standstill function that quickly reduces swaying. In environments where the wind can affect the plumb bob even if it has a standstill function, a windproof type plumb bob with a long, thin tube covering the plumb bob is suitable. - How to Retrieve the Line
There are two types of plumb bobs: one that manually winds up the thread when retrieving it, and one that automatically winds up the thread at the touch of a button.
Principle of Plumb Bobs
The plumb bob is hung down from a retainer fixed to a wall or post, and the distance from the wall or post is measured at two points: the top of the thread and the tip of the plumb bob. The vertical direction (verticality) is toward the center of the earth, and if the wall or post is vertical, the distance between these two points will be equal. Specifically, if the distance is the same at the top and bottom, the measured object is vertical. For accurate measurement, the distance should be as long as possible, but the longer the string, the harder it is for the weight to be stable.
When measuring, make sure that the plumb bob is not swaying. When the plumb bob is hanging down, it should swing in a circular motion and gradually decrease in amplitude. A plumb bob with a static function is designed to limit the direction of swaying to one direction so that the swaying will subside quickly.
The weight of the body of plumb bob is usually in the range of 100g to 1,000g. A plumb bob with a heavier weight will stop shaking faster. However, if a heavier weight is used than the weight compatible with the plumb bob retainer, the weight may fall, so the compatibility of the weight must be checked.
The swing retainer, thread, and plumb bob can each be replaced. When replacing the plumb bob, follow the manual for the plumb bob retainer or plumb bob, and make sure that the knot is securely attached so that it will not come undone.
To prevent injury from falling, it is recommended to wear a helmet, safety glasses, and safety shoes.
Structure and Function of Plumb Bobs
Plumb bobs are usually primitive, with a weight attached to the thread, but in recent years, more advanced products have been developed for the purpose of improving efficiency.
Automatic Take-up Type
In the automatic take-up type, the thread is stored inside the retainer and automatically taken up after measurement. The thread does not get tangled, allowing for quick and efficient operation. Some products are equipped with an automatic weight stop function. Normally, it takes time for the weight to stop, but the special construction allows the weight to stop quickly. This is especially useful outdoors where there is wind.
Dial Type
The dial type is a long metal plate equipped with a graduated measuring instrument. When the metal plate is aligned with the direction to be measured, the scale on the measuring instrument displays the angle.
Since the pendulum is inside the cover, it can check verticality and measure tilt without being affected by wind. To use the instrument, simply place the main unit against the object, and the needle rests more quickly and efficiently than with a typical plumb bob pendulum.