In an impact crusher, the apron gaps that determine product size are set by the apron adjustment mechanism. This mechanism consists of adjustment rods (tie rods) that hold the aprons in the required position, spring packs that push the apron back and let it open under overload, or, in modern crushers, hydraulic cylinders that replace them. When the mechanism does not work properly, the apron does not stay in the required position, chatters, or opens under overload and does not return; as a result product size fluctuates and the crusher is damaged.
In this guide we cover in detail mechanical and hydraulic apron adjustment systems, the adjustment steps, how overload protection works, failure signs, the maintenance schedule and spare part selection. For aprons and liners, see our apron liners guide, and for general impact crusher operation, our impact crusher guide.
Mechanical (Spring) Adjustment System
In the classic design, each apron hangs from one or two threaded adjustment rods that extend outside the housing. Turning the nut on the outer end of the rod moves the apron closer to or further from the rotor. A spring pack behind the apron or on the adjustment rod holds the apron in its set position, but lets it swing back when an uncrushable object arrives and then pushes it back into place.
- Advantages: Simple, economical, repairable on site.
- Disadvantages: Adjustment takes time; threads can seize with rust and material; spring preload falls over time.
Hydraulic Adjustment System
In hydraulic systems, the aprons are positioned by cylinders. The gap can be adjusted quickly from the control panel or with a hand pump; under overload, the hydraulic pressure is relieved so the apron opens, and it returns to its previous position once the load has passed. Some systems monitor wear and compensate for the gap automatically.
- Advantages: Fast, precise adjustment, safe overload protection, remote adjustment.
- Disadvantages: Requires hydraulic unit maintenance and checks of seals and accumulators.
| Feature | Mechanical spring | Hydraulic |
|---|---|---|
| Adjustment time | Long | Short |
| Adjustment precision | By thread pitch | Stepless |
| Overload response | Depends on spring preload | Controlled by pressure setting |
| Maintenance | Rod threads, springs, nuts | Cylinder, seals, hoses, oil |
Apron Gap Adjustment Steps
- Stop the crusher, confirm the rotor has stopped completely and apply LOTO.
- Open the housing or access through the inspection door; lock the rotor.
- Turn the rotor with the manufacturer’s device to bring the longest bar opposite the apron and measure the gap.
- On a mechanical system, turn the adjusting nuts equally on both sides to bring the apron to the target position; on a hydraulic system, adjust from the panel.
- Check the ratio of about 2:1 between the front and rear aprons.
- Tighten the lock nuts; compare spring length with the manufacturer’s value.
- Run the crusher; confirm the result with a sieve analysis of a product sample.
Failure and Wear Signs
- The apron not holding its set position, the gap changing by itself,
- Apron chatter and knocking,
- The apron not returning after an overload,
- Bending, crushed threads or rust on the adjustment rod,
- Broken or shortened springs,
- Leakage or pressure loss in the hydraulic cylinder.
Inspection and Maintenance Schedule
| Check | Interval | Method |
|---|---|---|
| Apron gaps | Weekly | Measurement |
| Spring length and condition | Weekly | Tape measurement, check for breakage |
| Adjustment rod threads | Monthly | Cleaning and lubrication |
| Hydraulic pressure and leaks | Daily | Pressure gauge and visual check |
| Apron hinge pins | Monthly | Clearance and wear |
Replacing Adjustment Rods and Springs
- LOTO is applied and the apron is safely supported.
- Spring preload is released in a controlled way.
- The adjustment rod and spring pack are removed; the rod bearings and the apron connection eye are checked.
- New rods and springs are fitted; a suitable lubricant is applied to the threads.
- The apron gap is set and the spring preload brought to the manufacturer’s value.
Common Mistakes
- Not adjusting both sides equally: The apron sits at an angle and wears excessively on one side.
- Adjusting without cleaning the threads: The rod seizes, and a forced nut crushes the threads.
- Running with a broken spring: The apron chatters and overload protection is weakened.
- Arbitrarily increasing hydraulic pressure: Overload protection loses its function.
Troubleshooting Table
| Symptom | Possible cause | Solution |
|---|---|---|
| Gap opens by itself | Loose lock nut, spring fatigue, hydraulic leak | Lock the nuts, renew springs/seals |
| Apron chatters | Low spring preload, incorrect ratio | Correct the springs and the ratio |
| Adjustment rod will not turn | Rust, trapped material, crushed thread | Cleaning, lubrication, rod replacement if necessary |
| Apron does not return after opening | Broken spring, accumulator failure | Check the springs or hydraulic system |
An Example from the Field
At an asphalt recycling plant, the front apron gap of an impact crusher was opening a little more every week and the product was getting coarser. The investigation found that in hot weather, bituminous material sticking to the aprons was filling the threads of the adjustment rods, the lock nuts could not be fully tightened and some of the springs were broken. The rods were cleaned and lubricated, the broken springs were replaced as a set and protective bellows were added to the adjustment rods. The gap drift stopped.
OEM or Aftermarket?
| Part | Check |
|---|---|
| Adjustment rod | Length, thread size, material strength |
| Springs | Free length, spring rate, wire diameter |
| Hydraulic cylinder | Stroke, bore, pressure rating, seal material |
| Nuts and washers | Thread compatibility and hardness |
Pre-Purchase Checklist
- Have the crusher model and adjustment system type been identified?
- Have adjustment rod dimensions and spring details been obtained?
- Are springs being ordered as a set?
- For a hydraulic system, has the need for a seal kit and accumulator been assessed?
Safety
- Compressed springs store a great deal of energy; they must not be released uncontrolled.
- In a hydraulic system, pressure must be relieved before work.
- The apron is heavy; it must be supported during removal.
Frequently Asked Questions
How often should the apron gap be adjusted?
A weekly check is recommended; more often for abrasive materials.
Can a mechanical system be converted to hydraulic?
Conversion kits exist for some models; it depends on the housing design.
Can springs be replaced one at a time?
For balanced preload, it is recommended to replace the springs on the same apron as a set.
How much does apron adjustment change the product?
Apron gaps directly affect the top size of the product; they should be assessed together with rotor speed.
How Does Overload Protection Work?
When an uncrushable object (a bucket tooth, steel, very hard rock) enters an impact crusher, the apron must retract to stop the object becoming trapped between it and the rotor. In mechanical systems, the spring pack compresses to let the apron open; once the object has passed, the spring pushes the apron back to its position. In hydraulic systems, when the pressure exceeds a set value the accumulator or relief valve comes into action, the apron opens and then returns to the set position.
If this protection does not work properly, the uncrushable object can break blow bars, damage the rotor and aprons, and even crack the housing. Spring preload or hydraulic pressure should therefore only be set to the manufacturer’s values.
Automatic Gap Adjustment
On some modern impact crushers, the apron gaps are adjusted automatically by the control system to compensate for blow bar and apron wear. To measure the gap, these systems move the apron towards the rotor in a controlled way, detect the contact point to establish a zero point and then retract it to the required gap. Automatic adjustment keeps product quality stable and shortens maintenance time; however, maintenance of the sensors and hydraulic system must not be neglected.
Effect of Operating Conditions on the Adjustment Mechanism
| Condition | Effect | Recommendation |
|---|---|---|
| Sticky material (asphalt, clay) | Build-up and seizing on the adjustment rods | Protective bellows, regular cleaning |
| Frequent tramp events | Spring fatigue, hydraulic seal wear | Metal separator, spring/seal checks |
| Humid environment | Rust | Lubrication and protective coating |
| Vibration | Nuts loosening | Lock nuts and locking |
Maintenance Record Form
| Field | Record |
|---|---|
| Front and rear apron gap | Weekly measurement |
| Spring length / hydraulic pressure | Measured value |
| Adjustment rod condition | Cleaning, lubrication, damage |
| Tramp events | Number and cause |
Stock Planning
- Keep one set of springs and adjusting nuts in stock for each apron.
- In hydraulic systems, seal kits and accumulators are critical spares.
- Adjustment rods should be held as spares against the risk of bending and thread damage.
More Questions
Does increasing spring preload solve chatter?
If the preload has fallen below the manufacturer’s value, correcting it reduces chatter; but going above the value weakens overload protection.
Can an automatic adjustment system be added to an older crusher?
Conversion solutions exist for some models; it depends on the housing and apron design.
Does apron adjustment affect product shape?
Yes. The gaps, and especially the front/rear ratio, change how crushing is distributed between the stages and so affect particle shape.
Spare Parts and Service for the Apron Adjustment System
CSP Mühendislik supplies apron adjustment rods, spring packs and hydraulic setting elements for impact crushers as part of its impact crusher spare parts range, and provides service support for apron adjustment and mechanism overhauls. Share your crusher model on WhatsApp for a quick quote.
