If your petrol lawnmower is vibrating excessively during use, this is usually caused by damage or imbalance in the cutting system. In most cases, this occurs after the blade has struck a hard object such as a tree root, paving slab, or drain cover.

This guide explains the most common causes and the checks you can carry out.

Table of Contents

  1. Common Cause of Excessive Vibration
  2. Blade Adaptor Inspection
  3. Blade Inspection
  4. Checking for a Bent Crankshaft

Common Cause of Excessive Vibration

Excessive vibration is most commonly caused by a blade strike, where the mower blade hits a solid object during operation.

When this happens, the engine may stop suddenly and force from the impact is transferred through the cutting system.

Typical causes of blade strike include:

  • Tree roots
  • Stones or rubble
  • Drain covers
  • Paving slabs
  • Uneven ground

Blade Adaptor Inspection

The blade adaptor is designed to absorb impact and protect the engine from damage.

Check for the Following:

  • Cracks in the cast aluminium body
  • Chips or missing sections
  • Missing shear pins
  • Signs of deformation

Action Required

If the blade adaptor is:

  • Cracked
  • Chipped
  • Missing shear pins
  • Broken or deformed

It must be replaced before the mower is used again.

Operating the mower with a damaged adaptor may lead to further mechanical damage.

Blade Inspection

In more severe impacts, the blade itself may be damaged.

Check the Blade for:

  • Bending or deformation
  • Visible dents or warping
  • Cracks or splits
  • Uneven or asymmetrical shape

Identifying Damage

A damaged blade will often appear visibly uneven when rotated by hand.

Action Required

If any damage is found, the blade must be replaced immediately.

Do not attempt to straighten or repair a damaged blade.

Checking for a Bent Crankshaft

The most serious consequence of a blade strike is a bent crankshaft.

Symptoms of a Bent Crankshaft:

  • Severe vibration during operation
  • Uneven rotation of the blade
  • Wobbling motion when the blade is turned by hand

How to Check:

  1. Ensure the engine is switched off and the spark plug cap is removed.
  2. Carefully rotate the blade by hand.
  3. Observe the movement of the blade and drive shaft.

If the crankshaft is bent, the engine will not rotate smoothly.

Action Required

A bent crankshaft is a major engine fault.

In most cases, the engine will require replacement rather than repair.