The beauty of trolley jacks is that they are generally much stronger, more reliable and safer to use than the old scissor jacks we often have in the back of our cars - but only if they are used properly.
From the outset it's important to stress that you should never attempt to get under a car with only a jack holding it up - it's essential to have a set of sturdy axle stands in place.
Trolley jacks are not intended to bear the weight of vehicles for an extended period of time - they are to be used solely to raise them off the ground. These jacks are very reliable, but they have been known to fail and could result in a serious accident if failure occurred when someone was underneath a vehicle.
Before raising your vehicle you should engage the handbrake and choose the transmission park position in an automatic car or the first gear position in a manual transmission vehicle. It's also wise to place a set of wooden or metal wheel chocks or wedges at the opposite end of the vehicle which you intend to raise - these will prevent your vehicle from rolling.
Once you've jacked your vehicle up with a floor jack you should then place two or four stands on the support points in place. Then you let the jack down and the jack stands will hold the vehicle steadily and safely. Ensure you always use matched pairs of stands.
Don't ever contemplate resting your vehicle on blocks or wood, bricks or any other solid object or you'll be asking for trouble - carrying out work under a vehicle often involves a lot of hammering and shaking, so there will likely be plenty of vibration and movement. Consequently there's the potential for wooden blocks to crack or slide and bricks may even shatter from the weight of a vehicle. The only accessory for the job is proper axle stands.
You should also ensure you have the right size jack for the weight of your car - thankfully most trolley hacks have a safety feature which will prevent them from attempting to raise a vehicle above the device's assigned weight limit. Don't ever attempt to force a jack to raise a vehicle that exceeds it's approved weight limit.
Once you have the vehicle jacked up and have the stands in place you should test the stability of your motor by giving it a little shake and pressing down on the bonnet.
It's also important to keep a trolley jack in prime condition by keeping it clean and adding oil to it when required. It's also worth getting a professional mechanic or other experienced auto enthusiast to give it the once over for you from time to time.
Always ensure your vehicle is on a completely solid and level surface before you attempt to raise it with a jack. You might also want to place a thick board underneath the jack if that would help prevent any risk of slipping. You should also check your vehicle manual to determine the exact position of the designated jack lifting points and axle stand support points.
The recommended front lifting point for most cars is the engine cradle's front member.The stand support locations are normally located just behind the front wheels and in front of the rear wheels. It you don't have a manual you can usually find the information you need online.
It makes sense to buy a trolley jack that's made by a company with a sound reputation for producing safe and reliable products. It's probably wise to steer clear of non-brand cheap imports, possibly made with inferior materials. The last thing you want is a jack with a slow leak that can impair the safe operation of its hydraulic acton. It's prudent to pay a bit extra a good quality jack, if only for peace of mind.
As with most things in life, you should follow you instincts and use your commonsense when it comes to safety. If you suspect something you are poised to to could be potentially risky, it probably is.
We are not qualified to offer professional advice about using a trolley jack safely and working under a vehicle - we are merely providing some general information. You should consult relevant safety procedure manuals and documentation to further your knowledge and seek professional advice if required.