I explain how to assess the straightness of a Automotive Lift Repair Tampa Florida hydraulic cylinder rod and determine the allowable run-out. This step is crucial in hydraulic cylinder repair because reusing a bent rod can compromise the rod seal’s lifespan and potentially waste the repair effort.
I also note that bent rods can often be successfully straightened and reused if they can be brought within the allowable tolerance. However, one of our members from a cylinder repair company disagrees with this approach:
In our experience, Automotive Lift Repair Tampa Florida ‘straightened’ rods are rarely truly straight. They typically end up with an S-curve after straightening attempts because they don’t re-bend in the same spot where they originally bent. Using these ‘straightened’ rods severely compromises the cylinder’s column strength, making it unsafe for heavily loaded applications. It also leads to uneven wear on the gland and rod bearing and poor sealing. Therefore, we avoid straightening or reusing bent rods and I suggest reconsidering your stance on this.
To clarify, when I refer to a “bent” rod, I mean one that has a deflection detectable only with a dial gauge. A rod is considered “bent” if its run-out exceeds allowable tolerance and “straight” if it falls within this tolerance.
In the past, I worked for a company that repaired cylinders from large mining excavators, with rod diameters of 200 millimeters (8 inches) and up, many of which were induction-hardened. Many of these rods arrived “bent” by my definition.
At that time, the cost to replace even the smallest of these Automotive Lift Repair Tampa Florida rods was around $10,000. Refusing to straighten them would have been financially beneficial initially, but we would have eventually lost business to competitors if customers realized our reluctance.
We provided a 6,000-hour warranty on our rebuilds, and any rod we straightened to our standards was expected to perform for at least 10,000 hours.
Automotive Lift Repair Tampa Florida Regarding induction-hardened rods, while I’ve heard third- or fourth-hand accounts of the hardening layer shattering during straightening, I’ve never experienced or verified this firsthand. My suspicion is that such incidents might involve rods bent more severely than defined here.
Nevertheless, straightening Automotive Lift Repair Tampa Florida rods bent as severely as an elbow is unwise, whether they are induction-hardened or not. Moreover, not all “bent” rods are suitable for straightening, and some should not be attempted.
With advanced sealing technology, the volumetric efficiency (i.e., leakage losses) of a well-maintained hydraulic cylinder can approach 100%. However, the mechanical-hydraulic efficiency of a cylinder varies depending on the type of seals and the tolerances between the piston rod and its wear bands.
Mechanical-hydraulic efficiency refers to the force lost due to mechanical and fluid friction. In a hydraulic cylinder, these losses arise from friction between the piston rod and its wear bands and seals, as well as from the friction of the fluid as it exits the return side of the cylinder at the required velocity. In practice, fluid friction is usually minimal, provided the cylinder’s ports and connections are properly sized.
For a single-rod cylinder, the mechanical-hydraulic efficiency is typically around 95% during extension and between 85% and 90% during retraction. This difference arises because mechanical and hydraulic friction remains nearly constant, but it represents a larger percentage of the available force during retraction due to the smaller effective area of the rod-end annulus.
For instance, consider a single-rod cylinder with a piston-to-annulus area ratio of 2:1. If the piston force during extension is 10,000 lbf (44 kN) and the annulus force during retraction is 5,000 lbf (22 kN), with constant losses due to mechanical and fluid friction of 500 lbf (2.2 kN), then:
In a hydraulic cylinder where volumetric efficiency is close to 100%, the overall efficiency is equal to the mechanical-hydraulic efficiency:
It’s uncommon, but we do occasionally receive seals returned due to dieseling failures. While identifying dieseling is relatively straightforward, explaining the exact cause and mechanism to the customer can be challenging.
As this member is aware, when air and oil are compressed in a hydraulic cylinder, they can ignite, leading to burning or even explosions. This results in mechanical damage to the cylinder and damage to its seals. The term ‘dieseling’ or ‘diesel effect’ refers to this combustion process, similar to what occurs in a diesel engine.
Air is usually the culprit and typically enters the hydraulic cylinder in one of two ways:
1. Past the Automotive Lift Repair Tampa Florida Rod Seal: When a double-acting hydraulic cylinder retracts under a load, the volume of oil required on the rod side can exceed what the pump supplies. This situation often arises due to a faulty or improperly adjusted load-control valve. Most rod seals are designed to contain high-pressure fluid but are not intended to prevent air from entering. As a result, a negative pressure can develop on the rod side of the cylinder, drawing air past the rod seal.
2. During Initial Installation: Air can also enter a hydraulic cylinder during the initial setup. To prevent this, it’s advisable to fill the cylinder chambers with clean hydraulic oil before connecting the hoses.
The key parameter for a hydraulic cylinder tube is its internal diameter, but it’s crucial to also assess its concentricity along the entire length. To evaluate this, use an internal micrometer to measure at least two perpendicular planes.
You need to accomplish two tasks: determine the nominal size of the tube and ensure this size is consistent along its length. If the internal diameter increases significantly at any point, the tube has ‘ballooned’ and should be discarded.
A standard internal micrometer is effective for Automotive Lift Repair Tampa Florida hydraulic cylinder tubes up to arm’s length. For longer or larger diameter cylinders, repair shops use a specialized bore gauge. This gauge features two wheels on one side and a spring-loaded pin on the other, with the pin’s position indicated on a dial gauge. Extension pieces are added as needed to measure the full length of the tube, forming an extended T shape. The readout is connected to the end of these extensions.
After setting the nominal bore diameter on the gauge, the operator moves it along the tube’s length while monitoring the readout for any deviations in diameter. This process is repeated in at least two perpendicular planes to check for ballooning or out-of-round conditions.
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