Editing Mechanical pencil (section)

==Mechanism types==
[[Image:Mechpencil05.jpg|thumb|A [[Pentel]] Sharp ratchet drafting pencil disassembled, showing three 0.5 mm graphite leads.]] 
[[Image:Staedtler 925-25 05 Mechanical Pencil.JPG|thumb|A [[Staedtler]] 925-25 05 assembled (top) and disassembled (bottom).]]Mechanical pencils can be divided into two basic types: those that both hold the lead and can actively propel it forward, and those that only hold the lead in position.

Screw-based pencils advance the lead by twisting a screw, which moves a slider down the barrel of the pencil. This was the most common type in the earlier part of the twentieth century. Many of these have a locking mechanism one way to allow the lead to be pushed back into the pencil.

A clutch pencil (or leadholder) tends to use thicker leads (2–5.6&nbsp;mm) and generally holds only one piece of lead at a time. A typical clutch pencil is activated by pressing the eraser cap on the top, to open the jaws inside the tip and allow the lead to freely drop through from the barrel (or back into it when retracting). Because the lead falls out freely when the jaws are opened, its forward movement cannot be controlled except by external means. This can be easily done by keeping the tip of the pencil a few millimeters above a work surface or the palm of one's hand. Some clutch pencils do have mechanisms which incrementally advance the lead, such as the Alvin Tech-Matic leadholder, but these are not normally considered to be in the same category as most pencils with propelling mechanisms.

Ratchet-based pencils are a variant of the clutch pencil, in which the lead is held in place by two or three small jaws inside a ring at the tip. The jaws are controlled by a button on the end or the side of the pencil. When the button is pushed, the jaws move forward and separate, allowing the lead to advance. When the button is released and the jaws retract, the "lead retainer" (a small rubber device inside the tip) keeps the lead in place, preventing the lead from either falling freely outward or riding back up into the barrel until the jaws recover their grip. Other designs use a precisely-fitted metal sleeve to guide and support the lead, and do not need a rubber retainer.

In one type of ratchet-based pencil (developed by Tombow Pencil Co.), [[Shaker mechanism|shaking]] the pencil back and forth causes a weight inside the pencil to operate a mechanism in the cap. A button may be present on the top or side of the pencil, to allow the user to advance the lead when necessary. Another variation advances the lead [[Auto-Advance mechanism|automatically]]. In this design, the lead is advanced by a ratchet but only prevented from going back into the pencil, just held from falling by a small amount of [[friction]]. The nib is a spring-loaded collar that, when depressed as the lead is worn away, extends out again when pressure is released.

An advanced ratchet type has a mechanism that rotates the pencil lead 9° counter-clockwise every time the lead is pressed on to the paper (which counts as one stroke), to distribute wear evenly. This ''auto-rotation'' mechanism keeps the lead 50% narrower than in the common propelling mechanical pencils, resulting in uniform thickness of the lines written onto the paper. The design was first developed by Mitsubishi Pencil Company, and named ''Kuru Toga'' under the Uni brand.<ref name="Kurutoga Website">[http://www.mpuni.co.jp/product/category/sharp_pen/kurutoga/index.html Kurutoga Website] Kuru Toga, which in Japanese, means "spinning to a taper".</ref> This type of pencil is most suited for Asian languages that have multiple strokes per letter or word, where the pencil is frequently lifted off the paper. The mechanism is not suitable for cursive writing used in western scripts. Another recent auto-rotation movement by Uni rotates the lead 18 degrees per stroke (or 20 strokes per complete revolution), which is better suited for western scripts.

There exist protection mechanisms that prevent the lead from breaking (within certain limits) when excessive pressure is exerted while writing. A mechanism employed in the ''DelGuard'' system by Zebra of Japan, causes the lead sleeve to extend outward when excessive pressure is applied at an angle. When excess vertical pressure is applied on the lead, the lead is automatically retracted inwards.