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As we all know, the hydraulic cylinder is one of the easiest components to understand in our hydraulic system. Even a person who has never been exposed to hydraulics can quickly have a basic understanding of the function of the hydraulic cylinder by looking at the schematic diagram and the actual object of the hydraulic cylinder.
The basic components of a hydraulic cylinder include the cylinder barrel tube, piston, piston rod, end cap, and seal. The piston rod not only serves as the connection between the hydraulic cylinder and the machine but also transmits the piston force. Therefore, the importance of the piston rod is paramount. This issue discusses hydraulic cylinder piston rod failures. When designing, selecting, and using piston rods, it’s crucial to understand their potential failure modes to better prevent these issues. The following are some of the most common problems with hydraulic cylinder piston rods.
1. The size of the piston rod is not suitable for the application
There are two common manifestations of an inappropriate piston rod size:
1) If the piston rod is too small, its strength will be reduced, especially in long-stroke applications. When the load exceeds the allowable value, the long and thin piston rod will bend or even break. This problem can be corrected in various ways, such as increasing the piston rod diameter, using stronger materials, or performing surface hardening treatments.
2) Conversely, the piston rod may be too large. Yes, you read that correctly, the piston rod can also be oversized. Many people may wonder, “A bigger rod is better—it’s stronger, won’t bend, and won’t break.” But in reality, a larger piston rod diameter isn’t necessarily better. While a larger diameter can increase piston rod strength, the larger area occupied by the rod increases the overall weight of the hydraulic cylinder and reduces the cylinder’s retraction force. Retraction force is equal to pressure multiplied by the effective area. If the rod is too thick, the remaining effective area within the cylinder barrel is naturally smaller. If the hydraulic cylinder is operating under tension, an oversized piston rod diameter will actually reduce the retraction force. An oversized piston rod is generally caused by improper design. Therefore, carefully calculate the retraction force of the cylinder based on your understanding of the actual operating conditions of the differential cylinder.
2. Piston rod thread broken
The threaded portion of the piston rod that screws into the piston can break for a variety of reasons. Selecting the incorrect thread for the application can be a contributing factor. Thread diameter plays a significant role in strength, so choosing a standard external thread when a stronger full-size thread is required can be problematic. Of course, the quality of the thread machining also impacts reliability. If the thread bend radius (the gap between the rod shoulder and its threads) is machined incorrectly or poorly, it can also be prone to breakage.
3. The piston rod is scratched and the seal is damaged
The piston rod surface can be scratched due to impurities in the hydraulic fluid, the ingress of hard foreign particles due to a failed wiper, or overtightening the guide sleeve. These surface scratches rarely affect the strength of the hydraulic cylinder, especially minor ones. However, dents and scratches on the rod can rub against the rod wiper and seal, damaging them over time.
Corrosion on the rod can also cause pitting, damaging the rod seal. Deterioration, damage, or improper installation of the seal can also cause leakage from the hydraulic cylinder. Using piston rods made of induction-hardened steel can prevent surface dents, and the appropriate thickness of the chrome plating must be selected. The thick chrome surface hardens the steel shell, providing critical corrosion resistance.
4. Piston rod corrosion
Because chromium protects against corrosion, if the chromium on the piston rod surface is removed during repair, the base metal of the piston rod is exposed to the elements. When hydraulic cylinders operate in humid environments or with harmful chemicals, corrosion or rust can occur. Therefore, it is essential to select the appropriate material for your application. For added rust resistance, piston rod materials are available in a variety of stainless steel alloys.
5. Mechanical damage
Industrial hydraulic cylinders rarely collide with objects or other machinery, but mobile machinery is susceptible to such damage. Falling rocks, metal, and other objects can strike the cylinder rod while the equipment is operating. No matter how thick the chrome plating or hardened surface layer, there’s no way to escape this kind of damage.
6. Piston rod deformation
The piston rod is prone to bending and deformation under long-term eccentric load or lateral force. This deformation can cause the hydraulic cylinder to move unsteadily, crawl or make abnormal noises, and in severe cases, may damage the seal.
In response to these problems with the hydraulic piston rod, it is recommended to regularly check the operating status of the hydraulic cylinder, maintain the cleanliness of the hydraulic oil, replace damaged seals in a timely manner, and perform necessary maintenance and repairs on the piston rod.
