The pitch diameters is an essential measurement for threaded parts because the pitch diameter determine how the two threaded parts will fit together and how they will carry a load. While many individuals focuses on the major diameter of the thread, which is the outermost measurement of the part with threads, the pitch diameter determines if a threaded component will fit with another part with the same thread type, if it will be able to carry the required load, and if other parts with the same thread will be able to be interchanged with this part. The pitch diameter is the measurement from the middle of the cylinder of the part where the width of the thread equal the width of the groove.
If the pitch diameter isnt correct by a few thousandths of an inch, it is possible that the part will experience binding or even strip on the threaded portion. The provided infographic shows a visual layout of the information regarding threads so that you can move from understanding the anatomy of a threaded part to understanding in what ways that part can be applied. There are eight different dimension for all parts with threaded portions.
What Pitch Diameter Is and Why It Matters
Additionally, the infographic makes clear the difference between the coarse inch series and fine inch series of threaded parts, as well as the different metric sizes that are used in manufacturing operations. Once you understand the various thread dimensions, you can use the formulas to calculate the pitch diameter of any given part that has a thread. There are two different type of threads that are used in industrial parts: coarse and fine threads.
Coarse threads are often used in construction equipment and other heavy equipment because they can tolerate some damage and quick assembly of the components. Fine threads are used in applications that require more tensile strength of the parts and where those components are subjected to vibration. For example, fine threads are often used on the components of an automobile cylinder head since those components will experience thermal cycling and require the maximum clamp load.
Coarse threads are used in larger structural metal brackets that must be assembled but where alignment of the holes is not perfect. Another parameter for threads is the thread class. Most commercial fasteners will use class 2A or 2B class threads because they strike a good balance between the cost of the fastener versus the required reliability of the fastener.
However, some applications require more fatigue life or sealing performance of the parts, but require class 3 fasteners because of the cost of those fasteners. Additionally, some of the largest equipment manufacturers use class 1 threads to provide the maximum clearance of the bolts in their components, even if that causes the fastener to become binding under dirty conditions. The infographic displays the tolerance zone for each class of thread so that each individual can understand how the 2A bolts may fit within the 2B nut.
The pitch diameter for parts can be measured on the shop floor using the three wire method. To utilize this method, you place wires of a specific diameter within the grooves of the threaded part. The outside diameter of the part is then measured over the wires with a micrometer tool.
The dimension of the part is calculated by subtracting a constant from the measured dimension. This method works because the wires of specific thickness will contact the threaded part exactly at the pitch diameter cylinder. However, the temperature of the part is also critical to measuring the pitch diameter because class 3 threads may fall outside of the tolerance if the temperature of the part is not maintained at around twenty degrees Celsius.
Go and no-go gauges can also be utilized to quickly determine if a threaded part passes the requirements for that component. The go gauge will have to fully enter the threaded portion of the component, but the no-go gauge will not advance the screw more than two turns into the component’s threaded portion. The infographic displays the different descriptions of go and no-go gauge measurements alongside the thread tolerance classes of the parts.
Thus, each individual will understand the reasons that the go member is of a size that allows it to fully enter the component and the no-go screw is of a size that will not allow it to advance more than a few turns into the component. When choosing a fastener, it is important to ensure that the pitch diameter will land within the tolerance of the requirements of the part that is to be assembled with the fastener. If the pitch diameter of the fastener is within the tolerance, then the part that is assembled with the fastener will develop its full strength.
The chart that is included in the infographic provides a visual example of each of these dimensions so that each individual doesnt have to memorize the calculations for each of the dimensions. When each individual understands the relationship between the major diameter, the pitch of the threads, and the pitch diameter, the individual will understand why the numbers associated with these dimensions are so important to industrial application of threaded parts.
