Crimping defects: how to spot rejects before shipment

The peculiarity of crimping rejects is that a defective joint often looks serviceable. It is assembled, it holds on the hose and even passes installation — and fails later, under pressure on the customer's machine. So quality control is built not on the impression "looks fine" but on specific signs. In this article we have collected the main defects and ways of detecting them in the shop, before shipment.

Visible defects: inspection and measurement

Under-crimp

The diameter after crimping is above the norm. Outwardly the joint may look neat, but the ferrule sits on the hose looser than designed: in mild cases it turns by hand, in severe ones the hose is pulled out under load. The causes are a wrongly chosen diameter, an error in the chart for the given hose and fitting pair, or die wear. How to determine and check the diameter correctly is covered in the article on crimp charts.

Over-crimp

The diameter is below the norm. The signs are visible on inspection: a bead of rubber is extruded at the ferrule end, the ferrule shows a noticeable narrowing, and inside the joint the braid may be cut by the ferrule edge. A cut braid is the most dangerous consequence: the joint holds during installation and for a while in service, but the broken wires carry no load, and under pressure pulsation the hose fails. Over-crimping cannot be corrected — such a joint is cut off and reassembled.

Ferrule misalignment

The ferrule is crimped out of line with the hose. The usual cause is a hose inserted into the fitting not to the stop or at an angle; less often, incomplete closing of the dies due to contamination of the crimping head. It is checked by sighting along the joint and by the uniformity of the gap between ferrule and hose.

Incomplete hose insertion

The hose is inserted into the fitting to less than full depth, and the ferrule is crimped over only part of the reinforcement. The defect is not visible from outside, so it is prevented before crimping: an insertion depth mark is placed on the hose and its position at the ferrule end is checked after assembly.

Uneven flats

A noticeable difference between two diameter readings turned by 90°, unequal width of the die flats, scores on the ferrule. These are signs not of a one-off defect but of the machine's condition: wear of the die working faces or contamination of the die seats. How the die surface affects the joint is the subject of a separate article.

Hidden defects: pressure monitoring on CNC machines

Part of the rejects is not visible from outside at all: a cracked ferrule, a cracked nipple, a destroyed hose layer inside the joint. This is where the automatic control built into numerically controlled machines comes in.

The control system monitors the hydraulic pressure throughout the crimping cycle. Normally the pressure curve rises smoothly and repeats from cycle to cycle. A sharp pressure change in the middle of the cycle means something has happened in the joint: the ferrule has cracked, the nipple has cracked, an element inside has failed. In this case quality control stops the machine so that the operator can inspect the joint and decide — to production or to rejects.

Such monitoring does not replace inspection and measurement, but it covers exactly the zone that inspection cannot reach. For serial production it is also statistics: the cycle log shows on which pairs and batches failures occur more often.

Outgoing inspection: a short procedure

1. Inspection: uniformity of the ferrule, no rubber bead or scores, coaxiality.
2. Diameter measurement at the ferrule centre, two readings turned by 90°.
3. Check of the insertion depth mark at the ferrule end.
4. For critical assemblies — pressure testing on a bench with a report.

The four points take less than a minute per assembly and remove the overwhelming majority of claims.