Electric Motor and Gearbox Selection for a Screw Sand Washer

On a screw sand washer, the motor and gearbox are selected together: the motor supplies the power, and the gearbox produces the torque needed to turn the screw slowly through water and sand. What matters on this machine is that the screw turns at a very low speed, that sand settling at the bottom of the tank buries the screw after a stop, and that the motor works in a place with constant splashes of water and slurry. Below we go through these three topics and motor-gearbox matching step by step.

How does a sand washer work?

A screw sand washer consists of a long screw turning inside an inclined tank. Sand and water are fed into the lower part of the tank. As the screw turns, it carries the sand up the slope; meanwhile clay, silt and fine dust flow out over the overflow weir with the water. The sand that reaches the upper end of the tank is discharged after giving up part of its water.

This job needs the screw to turn slowly: a fast-turning screw churns the water, mixes fine material back into the sand and spoils dewatering. The right screw speed is set by the machine manufacturer. Twin-screw models with two screws side by side are also common for higher capacity.

The bucket-wheel washer, another type used for sand washing, has a different drive arrangement; this article focuses on the screw type.

Drive train: motor, belt and gearbox

The low screw speed is achieved by greatly reducing the roughly 1,450 rpm coming from the motor. The three arrangements most often seen on site are:

• Shaft-mounted gearbox and belt: The gearbox is slid onto the end of the screw shaft and fixed to the tank with a torque arm. The motor sits on a base above or beside the gearbox and drives it by V-belt. The pulley ratio allows small adjustments to screw speed.
• Flange-mounted motor and gearbox: The motor is connected directly to the gearbox input flange. The layout is compact, but there is no pulley option for speed adjustment.
• Foot-mounted motor, coupling and gearbox: Motor and gearbox stand on a common frame with a flexible coupling between them. This is found on large machines.

On twin-screw machines each screw may have its own motor and gearbox, or a single motor may feed two gearboxes through an intermediate shaft. Knowing which arrangement is used is the first step in choosing a spare motor.

Gearbox ratio and screw speed calculation

Screw speed is found by dividing the motor speed first by the pulley ratio and then by the gearbox ratio: screw speed = motor rated speed × (motor pulley ÷ gearbox pulley) ÷ gearbox ratio.

A hypothetical calculation

The numbers are chosen only to show the method. Consider a 4-pole motor (rated 1,465 rpm), a 160 mm motor pulley, a 250 mm gearbox pulley and a 1:40 shaft-mounted gearbox. The speed at the gearbox input is 1,465 × 160 ÷ 250 ≈ 938 rpm, and the screw speed is 938 ÷ 40 ≈ 23 rpm. If a 6-pole motor (about 970 rpm) were used in the same arrangement, the screw would drop to about 15 rpm; this difference changes washing and dewatering behaviour.

The conclusion is clear: the spare motor must have the same number of poles as the old one. If the screw speed needs to change, this is done with the pulley ratio and within the range recommended by the manufacturer.

Matching gearbox power and motor power

The gearbox also has a power and output torque limit. Moving the motor up one power size only makes sense if it can be done without exceeding this limit. The ratio, input power and service factor on the gearbox nameplate should be read before starting motor selection. General information on geared applications is on the electric motors for gearboxes page.

Starting with settled sand

The hardest moment on a sand washer is the first movement after a long stop. When the machine stops, the sand in the tank settles in the water and buries the lower part of the screw. When the screw is to be turned again, the motor has to move this packed mass of sand. A starting torque noticeably higher than the normal running load is required.

The main operating measures taken for this situation are:

• Stopping after emptying: After the feed is cut, the screw is kept turning for a while; the machine is stopped with less sand in the tank.
• Lifting device: On some machines the lower end of the screw can be raised by a hoist or hydraulic system. Lifting the screw out of the settled sand before starting protects the motor.
• Loosening with water: Adding water to the tank before starting loosens the sand and reduces resistance.
• Suitable starting method: With star-delta starting the starting torque drops, so the motor may not start with a buried screw. A soft starter or controlled start with a drive may give better results; the torque requirement is taken into account when choosing the method.

If the protection keeps tripping during starting, the solution is not to bypass it but to review why the sand settles and the starting method. Repeated forcing wears the motor winding and the gearbox gears.

Water, slurry and open sites

The sand washer motor is a typical example of a drive working in a wet environment. Water splashing from the tank, sandy slurry and wash-water mist surround the motor, and the machine is usually on an open site.

• HEM Motor's motors on sale have IP55 protection and class F insulation. This protection is against dust and water splashes; washing the motor with a pressure jet or leaving it in slurry is outside the scope of this rating.
• To protect the motor from direct splashes, a shield can be fitted above and beside it that does not block the fan's air intake.
• In a belt arrangement, the belt and pulleys must also be protected from splashes; a wet belt slips and heats up.
• The sealing of the terminal box cover and cable gland should be checked regularly.
• On machines that stand idle for long periods, moisture can condense inside the motor; an insulation check before the first start of the season is recommended.

Cast iron frames are often preferred in wet, vibrating sand pits for their durability. Other topics for quarries and crushing-screening plants are on the stone crushing plant electric motors page.

You can see HEM Motor models suitable for sand washer drives, with their prices, on the buy electric motors page.

Example cast iron models for sand washers

Motor power is set by the machine manufacturer according to screw diameter, tank length, inclination and capacity. The exact value is read from the nameplate of the machine or the existing motor. As examples, the three models below are 4-pole, cast iron frame and rated 400/690 V (Δ/Y):

• 132S-4 (5.5 kW): rated 1,460 rpm, IEC 132S frame, 10.7 A at 400 V, rated torque 36 Nm.
• 160M-4 (11 kW): rated 1,465 rpm, IEC 160M frame, 20.4 A at 400 V, rated torque 71.7 Nm.
• 160L-4 (15 kW): rated 1,465 rpm, IEC 160L frame, 27.3 A at 400 V, rated torque 97.8 Nm.

160M and 160L share the same shaft height but have different foot spacings; when moving from 11 kW to 15 kW the base holes are checked. In a flanged arrangement, it is confirmed with the order that the gearbox input flange fits the new frame. Prices are kept up to date on the relevant product pages.

Conclusion and information needed for a quotation

Choosing the right spare motor for a screw sand washer means keeping the same number of poles, respecting the gearbox power limit and easing starts with settled sand through operating measures. To speed up the quotation, prepare the following:

• Photos of the motor nameplate and the gearbox nameplate
• Drive arrangement: shaft-mounted gearbox and belt, flanged connection or coupled frame
• Single or twin screw, and the machine make and model
• Starting method and any problems when starting after a stop

You can reach the list of suitable models and prices through our online sales section. Send us the nameplate photo, speed and mounting arrangement of the motor on your sand washer via the nameplate submission page, and we will prepare a quotation with the right model for you.

Frequently Asked Questions

Does turning the screw faster increase capacity?

Not always. A fast-turning screw churns the water, mixes fine material back into the sand and spoils dewatering; speed should be kept within the range recommended by the manufacturer.

Why does the motor struggle at the morning start?

Overnight the sand settles at the bottom of the tank and buries the screw. Emptying the tank before stopping, adding water before starting or lifting the screw reduces the starting load.

Can I choose two motors of different power on a twin-screw machine?

Not recommended. Since both screws carry the same load, the motors and gearboxes should be identical; otherwise maintenance and spare parts management also become harder.

Can I upsize the motor without changing the gearbox?

Only if the input power and output torque on the gearbox nameplate allow it. Otherwise a stronger motor pushes the gearbox beyond its design limit.