Do you know this feeling? You're assembling a new system, tightening a hose to the distributor, the wrench offers solid resistance – everything seems perfectly fitted. You start the machine, and oil immediately begins to drip from under the nut.
What do many home mechanics do in this situation? They reach for Teflon tape or plumber's hemp, wrap the thread thickly, and tighten everything again "by force." The result? At best, the oil still drips. At worst – shreds of Teflon fall into the system, clogging nozzles in the distributor and damaging the pump.
BRhydraulic experts explain why power hydraulics is not like bathroom plumbing, how high-pressure seals work, and how to eliminate leaks once and for all.
1. Forget about hemp: How is hydraulics really sealed?
In a domestic water installation (where the pressure is only 3-4 bar), sealing is done on the thread itself (using hemp or Teflon threads).
In agricultural and construction machinery, we are dealing with pressures of 200-300 bar. No Teflon can withstand this. In power hydraulics, the thread serves only to mechanically pull two elements together. The sealing itself is done completely differently:
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Cone seal: The inside of the fitting and the hose end have specially profiled, smooth cones (most often at an angle of 60° or 24°). When you tighten the thread, metal presses against metal with such powerful force that oil cannot possibly penetrate this barrier.
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O-ring seal (flat): Some fittings (e.g., in ORFS standard or with flanges) have a groove with a rubber seal that is flattened during tightening.
Conclusion: If oil leaks after tightening the hose to the fitting, it means that the cones are scratched, cracked, or... you are trying to combine two completely different thread standards.
2. Metric vs. Imperial (BSP) Threads – The Most Common Mistake
"Sir, the thread only went halfway and then stopped, and then it started leaking." This is a classic symptom of mixing the two most popular standards on the market.
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Imperial BSP Threads (British): These are the absolute king in agriculture and Western machinery (standard designations include 3/8", 1/2", 3/4"). They have a sealing cone with a 60° angle.
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Metric Threads (M): They dominate older Polish machines (e.g., Bizon, Ursus), Russian equipment, and German standards (DIN). They are designated by diameter and pitch (e.g., M18x1.5, M22x1.5). They usually have a 24° cone.
Never connect a hose with a metric thread to an imperial fitting! Sometimes these threads are so close in diameter (e.g., M22x1.5 and 5/8" BSP) that they can be screwed on for two or three turns, damaging each other and guaranteeing a leak. If you are not sure what you have in your machine – use a caliper and a thread gauge.
3. How to screw a fitting into a pump body or distributor? (ED washers)
Tightening a hose to a fitting is one thing, but how do you seal the fitting (connection) itself into the cast iron block of a pump, cylinder, or distributor? There are no cones here.
It is a mistake to use ordinary copper washers (which need to be tightened very hard, often resulting in stripping the thread from an aluminum pump). The solution used by professionals are metal-rubber washers (so-called ED / Dowty seals). This is a steel ring with vulcanized rubber inside. Simply place it on the fitting and lightly tighten it with a wrench to the body. The rubber perfectly fills microscopic gaps, and the steel ring prevents it from tearing under high pressure. This guarantees 100% sealing and a dry valve block.
Summary
A clean and dry machine means a lower risk of fire, savings on oil top-ups, and no pressure problems. Throw out the hemp and Teflon tape from your workshop. If you're measuring an old thread with calipers and don't know what it is – send us the dimensions, and we'll select the right fitting!
