In the world of industry and manufacturing, many machines and applications require a pushing / pulling force in a straight line or arc. For this purpose, we often use hydraulic cylinders, which serve as the muscles of our machinery and equipment.
The hydraulic cylinder is a piston and rod assembly inside a cylindrical bore, actuated by the flow and pressure of hydraulic fluid with the output of linear motion and mechanical force.
We find hydraulic cylinders in industrial machines like presses, jacks and plastic molding equipment, as well as mobile equipment like excavators and mining trucks.
Hydraulic cylinders are useful in their simplicity, durability and significant power density.
A Few Basic Cylinder Types
Tie Rod Cylinders
Made of high strength steel, often seen in automotive manufacturing and general industrial applications.
Welded Or Mill Type Cylinders
Originally designed for use in steel mills, foundries, and other heavy-duty jobs such as civil engineering and marine applications in harsh environments with wide temperature ranges.
These are single-acting cylinders used for pushing applications such as heavy-duty presses and hydraulic jacks.
Factors To Consider For Cylinder Selection
- Bore size
- Rod size
- Stroke length
These measurements are mainly based on the size of the load to be moved, and the available system pressure, or operating pressure. Common operating pressures range from 500 to 10000 psi.
We first calculate the mass of the load to be moved. Once we know the mass, we can figure out the force required to move it. This will depend on the friction and acceleration factors of our specific application. In the calculation of our force requirement, we must always include a buffer to allow for a margin of error.
Once we know our force requirements, we can calculate the bore size requirement:
Load to be moved (pounds) / Operating pressure (psi) = Area (square inches)
By converting the area to a diameter, we can determine the bore size of the cylinder required to move the load.
To account for system losses, one should generally size the cylinder at 15 – 20% above the load requirement.
Rod Selection Recommendations
Our next step is to select an appropriate rod size.
Piston rod diameter is critical. An underspecified piston rod makes the cylinder more prone to stress, wear, and failure.
To select an appropriate rod size, we must know the length of the stroke. For cylinders with long strokes, a fully extended rod can buckle under its own weight. Rod deflection should never exceed 1 to 2mm
Here at Kappa Engineering, we use software to calculate rod buckling strengths for given rod sizes and use the data to make suitable recommendations for given loads.
- Standard rod options: For a given bore, only use the smaller rod for short-stroke push loading or reduced pressure applications. Use the larger rod to obtain maximum reliability and lifetime.
- If the required rod diameter exceeds that of the largest available, one should reconsider design parameters or look into a custom made cylinder.
Geometry Of The Installation
Once we know the nature of the load, we then need to consider the geometry involved in moving it. The structure of the machine or installation will determine the type of mount to be used.
For a machine such as a ‘jack’ or a ‘press’, the actuation is directly up and down. For some other machines, the center of the load is not centered at perpendicular angles to the point of lift force, and the cylinder force requirement changes.
Mounting plays an important role in performance and the main factor is whether the cylinder body is stationary or pivots.
Fixed Mount options are best for straight-line force transfer and minimal wear, they include:
- Centerline mount
- Flange mount
- Side mount
The fixed mount is easy to install and service, but has little tolerance for misalignment and so must be well guided.
Pivot Mount options absorb force on the cylinder centerline and allow for one plane of rotation. They generally include:
- Clevis mount
- Trunion mount
- Spherical-bearing mount
The pivot mount is applicable wherever the cylinder needs to change alignment during motion.
Seals are a vulnerable aspect of the hydraulic system. Choosing the correct seal will increase cylinder life and prevent fluid leakage. To choose the correct seal we must understand the cylinder operating conditions and temperatures, as well as the type of hydraulic fluid used.
Polyurethane seals – Most standard applications – flexibility, strength and wear resistance.
Teflon seals – Highly dynamic applications and systems requiring low friction.
Buna-N(Nitrile) seals – Cylinders using water, or water-glycol based fluids.
Fluorocarbon seals – Systems using synthetic fluids and or high temperature operating environments.
Depending on your application, cushions at the end of the cylinder stroke may be required. Cushions are recommended to decelerate high-speed rods and buffer the impact of the piston assembly on the cylinder end cap. Cushions are optional and do not affect the cylinder envelope or mounting dimensions.
Other options to prevent impact loads are external mechanical stops or proportional valve technology.
We hope that this information was helpful as an introduction to choosing the correct solutions for your cylinder requirements.
At Kappa Engineering, our technical capacity and strategic partnerships enable us to repair and manufacture any size and quantity of hydraulic cylinders. We have a dedicated repairs department that ensures minimal downtime. Our advanced quality control system is TUV certified ISO 9001:2015 to make sure our newly manufactured products and repairs conform to the highest standards.