A prismatic joint is a connection between two objects that allows relative motion along a single axis. Motion that’s perpendicular to this axis (or rotation about any axis) is prevented. This means the joint has one degree of freedom (1-DOF).
A prismatic joint allows only linear motion along a single axis.
A prismatic joint is often called a slider — as for example a “crank-slider linkage.” This is a type of kinematic pair and an idealized way to describe how motions are constrained within machines.
There are several physical embodiments of a prismatic joint, such as slideways. Rectangular cross-sections may be used in combination with plain or rolling bearings to enable linear motion while preventing rotation about the axis of movement.
Hydraulic and pneumatic cylinders are typically modeled as prismatic joints although they may also permit rotation about their axis. Kinematic pairs are classified as higher pairs and lower pairs. Higher pairs involve point or line contact, such as a ball or roller that’s rolling over a surface. Lower pairs can be visualized as surface contact.
The types of lower pair are:
• Prismatic joint (1-DOF): A joint that only allows linear motion along a single axis.
• Revolute joint (1-DOF): A joint that only allows rotational motion about a single axis.
• Cylindrical joint (2-DOF): A combination of a prismatic and a revolute joint. This joint allows only linear motion along an axis and rotation about that axis.
• Spherical joint (3-DOF): A joint that ensures two bodies remain connected at a common point, preventing any linear translation. However, rotation about any axis is permitted. A ball and socket joint is one example.
• Planar joint (3-DOF): A joint that only allows translation over a plane and rotation about an axis normal to this plane. This type of joint is produced by a stable object resting on a flat surface.
• Screw pair (1-DOF): A joint that constrains motion to a helical path, such as a nut on a bolt. Although both translation and rotation occur, they are coupled so that there is only on degree of freedom – position along the helix.
Both prismatic and revolute joints may be considered special cases of the screw pair. The prismatic joint has a helix angle of 90 degrees and the revolute joint has a helix angle of zero. Within real machines, higher pairs may exist at a low level, for example within roller bearings.
However, when analyzing a mechanism we normally consider a higher level so that the entire roller bearing would be considered as a revolute joint. The design and analysis of machines is generally concerned with lower pairs. The design and analysis of machine elements such as bearings and gears involves higher pairs.
Filed Under: Fastening + joining